View source for Film speed

{{Short description|Measure of a photographic film's sensitivity to light}}
{{distinguish|frame rate}}
{{redirect|Slow film|the genre of films|slow cinema}}
{{Use dmy dates|date=June 2019|cs1-dates=y}}
[[File:Provia 100F RDPIII Box with ISO film speed.jpg|thumb|right|This film container denotes its speed as ISO&nbsp;100/21°, including both arithmetic (100&nbsp;ASA) and logarithmic (21&nbsp;DIN) components. The second is often dropped, making (e.g.) "ISO&nbsp;100" effectively equivalent to the older ASA speed. (As is common, the "100" in the film name alludes to its ISO rating.)]]
'''Film speed''' is the measure of a [[photographic film]]'s [[photosensitivity|sensitivity to light]], determined by [[sensitometry]] and measured on [[#Film|various numerical scales]], the most recent being the [[#ISO|ISO]] system introduced in 1974. A closely related system, also known as ISO, is used to describe the relationship between exposure and output image [[lightness]] in digital cameras. Prior to ISO, the most common systems were [[#ASA|ASA]] in the United States and [[#DIN|DIN]] in Europe.

The term ''speed'' comes from the early days of photography. [[Photographic emulsion]]s that were more sensitive to light needed less time to generate an acceptable image and thus a complete exposure could be finished faster, with the subjects having to hold still for a shorter length of time. Emulsions that were less sensitive were deemed "slower" as the time to complete an exposure was much longer and often usable only for [[Still life|still life photography]]. Exposure times for photographic emulsions shortened from hours to fractions of a second by the late 19th century.

In both film and digital photography, choice of speed will almost always affect image quality. Higher sensitivities, which require shorter exposures, typically result in reduced image quality due to coarser [[film grain]] or increased [[Image noise|digital image noise]]. Lower sensitivities, which require longer exposures, will retain more viable image data due to finer grain or less noise, and therefore more detail. Ultimately, sensitivity is limited by the [[quantum efficiency]] of the film or sensor.

To determine the exposure time needed for a given film, a [[light meter]] is typically used.

==Film speed measurement systems{{anchor|Film|Film speed}}==
<!-- This section is linked from inside or outside this article, preferable by "#Film". Do not rename without keeping in sync. -->

===Emulsion speed rating criteria===
Five criteria for the rating of emulsion speed have been used since the late 19th century, listed here by name and date, these criteria are: threshold (1880), inertia (1890), fixed density (1934), minimum useful gradient (1939) and fractional gradient (1939).<ref>{{citation
 | last = Allbright | first = G S
 | journal = The Journal of Photographic Science
 | doi = 10.1080/00223638.1991.11737126
 | title = Emulsion Speed Rating Systems
 | year = 1991| volume = 39
 | issue = 2
 | pages = 95–99
 }}
</ref> 
====Threshold====

The threshold criterion is the point on the [[Sensitometry|characteristic curve]] corresponding to just perceptible density above fog.

====Inertia====

The inertia speed point of an emulsion is determined on the Hurter and Driffield [[Sensitometry|characteristic curve]] by the intercept between the gradient of the straight line part of the curve and the line representing the base&nbsp;+&nbsp;fog (B+F) on the density axis. 

====Fixed density====

The fixed density speed point is determined by defining a fixed minimum density as the basis of the emulsion speed (e.g. 0.1 above B+F).

====Minimum useful gradient====

The minimum useful gradient criterion places the speed point where the gradient first reaches an agreed value (e.g. tan <big>𝜃</big> = 0.2).

====Fractional gradient====

The fractional gradient is defined as the speed point at which the slope of the characteristic curve first reaches a fixed fraction (e.g. 0.3) of the average gradient over a range (e.g. 1.5) of the characteristic curve.<ref>{{citation
 | last = Jacobson | first = Ralph E
 | publisher = Focal Press 
 | title = The Manual of Photography (Seventh Edition)
 | page = 410
 | year = 1978}}
</ref>

===Historical systems===

====Warnerke{{anchor|Chapman Jones|Henry Chapman Jones|Jones}}====
<!-- This section is linked from inside or outside this article, preferable by "#Warnerke". Do not rename without keeping in sync. -->
The first known practical [[sensitometry|sensitometer]], which allowed measurements of the speed of photographic materials, was invented by the Polish engineer [[Leon Warnerke]]<ref name="DIN_1934_DIN-4512_Introduction"/>&nbsp;– pseudonym of {{lang|pl|[[Władysław Małachowski]]}} (1837–1900)&nbsp;– in 1880, among the achievements for which he was awarded the [[Progress Medal (RPS)|Progress Medal]] of the [[Photographic Society of Great Britain]]<!-- Today named: Royal Photographic Society (RPS) --> in 1882.<ref name="RPS_2011_Progress_Medal_1882"/><ref name="Jones_1911_Cyclopaedia"/> It was commercialized since 1881.

The Warnerke Standard Sensitometer consisted of a frame holding an opaque screen with an array of typically 25 numbered, gradually pigmented squares brought into contact with the photographic plate during a timed test exposure under a [[Phosphorescence|phosphorescent]] tablet excited before by the light of a burning [[magnesium]] ribbon.<ref name="Jones_1911_Cyclopaedia"/> The speed of the emulsion was then expressed in 'degrees' Warnerke (sometimes seen as Warn. or °W.) corresponding with the last number visible on the exposed plate after development and fixation. Each number represented an increase of 1/3 in speed, typical plate speeds were between 10° and 25° Warnerke at the time.<!-- According to Josef Maria Eder - History of photography: p. 451: 20° Warnerke = 10° (European) Scheiner, 24-25° Warnerke = 16-18° (European) Scheiner. -->

His system saw some success but proved to be unreliable<ref name="DIN_1934_DIN-4512_Introduction"/> due to its spectral sensitivity to light, the fading intensity of the light emitted by the phosphorescent tablet after its excitation as well as high built-tolerances.<ref name="Jones_1911_Cyclopaedia"/> The concept, however, was later built upon in 1900 by [[Henry Chapman Jones]] (1855–1932)<!-- 1932-03-07 --> in the development of his<!-- So called Chapman Jones --> plate tester and modified speed system.<ref name="Jones_1911_Cyclopaedia"/><ref name="Hasluck_1905_Jones"/>

====Hurter &amp; Driffield{{anchor|H&D}}====
<!-- This section is linked from inside or outside this article, preferable by "#H&D". Do not rename without keeping in sync. -->
Another early practical system for measuring the sensitivity of an emulsion was that of [[Hurter and Driffield]] (H&D), originally described in 1890, by the Swiss-born [[Ferdinand Hurter]] (1844–1898) and British [[Vero Charles Driffield]] (1848–1915). In their system, speed numbers were inversely proportional to the exposure required. For example, an emulsion rated at 250&nbsp;H&D would require ten times the exposure of an emulsion rated at 2500&nbsp;H&D.<ref name="Sowerby"/>

The methods to determine the sensitivity were later modified in 1925 (in regard to the light source used) and in 1928 (regarding light source, developer and proportional factor)—this later variant was sometimes called "H&D&nbsp;10". The H&D system was officially<ref name="Konovalov_2007"/><!-- TBD: I found documents stating that there was no standard before 1945, so we should find some historic documents stating that H&D (or whatever) was really an *official* standard in the USSR, and not just semi-standard. Also, since GOST 2817-45 was apparently first defined in 1945, why was the GOST standard not introduced before 1951, when its revision GOST 2817-50 was already in place? Runner1616: Until 1951 both standards existed simultaneously in USSR. Any photo book before 1950 operated with H&D (Х и Д) --> accepted as a standard in the former [[Soviet Union]] from 1928 until September 1951, when it was superseded by [[#GOST|GOST]]&nbsp;2817–50.

====Scheiner{{anchor|Scheinergrade|Sch|Sch.}}====
<!-- This section is linked from inside or outside this article, preferable by "#Scheiner". Do not rename without keeping in sync. -->
The ''Scheinergrade'' (Sch.) system was devised by the German astronomer [[Julius Scheiner]] (1858–1913) in 1894 originally as a method of comparing the speeds of plates used for astronomical photography. Scheiner's system rated the speed of a plate by the least exposure to produce a visible darkening upon development. Speed was expressed in degrees Scheiner, originally ranging from 1° to 20°&nbsp;Sch., with each increment of a degree corresponding to a ''multiplicative'' factor of increased light sensitivity. This multiplicative factor was determined by the constraint that an increment of 19° Sch. (from 1° to 20°&nbsp;Sch.) corresponded to a hundredfold increase in sensitivity. Thus emulsions that differed by 1° Sch. on the Scheiner scale were <math>\sqrt[19]{100} = 1.2742...</math>-fold more (or, less) sensitive to each other. An increment of 3°&nbsp;Sch. came close to a doubling of sensitivity<ref name ="Sowerby"/><ref name="Riat_2006"/> <math>(\sqrt[19]{100})^3 = 2.06914...</math>.

The system was later extended to cover larger ranges and some of its practical shortcomings were addressed by the Austrian scientist [[Josef Maria Eder]] (1855–1944)<ref name="DIN_1934_DIN-4512_Introduction"/> and Flemish-born botanist {{Interlanguage link multi|Walter Hecht|de|3=Walter Hecht}} (1896–1960), (who, in 1919/1920, jointly developed their ''Eder–Hecht neutral wedge sensitometer'' measuring emulsion speeds in ''Eder–Hecht'' grades). It remained difficult for manufacturers to reliably determine film speeds, often only by comparing with competing products,<ref name="DIN_1934_DIN-4512_Introduction"/> so that an increasing number of modified semi-Scheiner-based systems started to spread, which no longer followed Scheiner's original procedures and thereby defeated the idea of comparability.<ref name="DIN_1934_DIN-4512_Introduction"/><ref name="Sheppard_1932_Dresden-Congress"/>

Scheiner's system was eventually abandoned in Germany, when the standardized [[#DIN|DIN]] system was introduced in 1934. In various forms, it continued to be in widespread use in other countries for some time.

====DIN{{anchor|DIN 4512|DIN 4512-1|DIN 4512-4|DIN 4512-5}}====
<!-- This section is linked from inside or outside this article, preferable by "#DIN. Do not rename without keeping in sync. -->
The DIN system, officially DIN standard 4512 by the ''{{lang|de|[[Deutsches Institut für Normung]]}}'' (then known as the ''{{lang|de|Deutscher Normenausschuß}}''<!-- Sic, old German spelling, no typos here. --> (DNA)), was published in January 1934. It grew out of drafts for a standardized method of sensitometry put forward by the ''{{lang|de|Deutscher Normenausschuß für Phototechnik}}''<!-- Sic, old German spelling, no typos here. --><ref name="Sheppard_1932_Dresden-Congress"/> as proposed by the committee for sensitometry of the ''{{lang|de|Deutsche Gesellschaft für photographische Forschung}}''<ref name="Biltz_1933_DIN-Grade"/> since 1930<ref name="Heisenberg_1930_Tagung"/><ref name="Voss_2002_Luther"/> and presented by {{Interlanguage link multi|Robert Thomas Dietrich Luther{{!}}Robert Luther|de|3=Robert Thomas Dietrich Luther}}<ref name="Voss_2002_Luther"/><ref name="Buckland_2008_Kinamo"/> (1868–1945)<!-- 1868-01-02 (21. December 1867 in Julian caldendar) to 1945-04-17, chemist. --> and [[Emanuel Goldberg]]<ref name="Buckland_2008_Kinamo"/> (1881–1970) at the influential VIII. [[International Congress of Photography]] (German: ''{{lang|de|Internationaler Kongreß für wissenschaftliche und angewandte Photographie}}''<!-- Sic, old German spelling, no typos here. -->) held in [[Dresden]] from 3 to 8 August 1931.<ref name="Sheppard_1932_Dresden-Congress"/><ref name="Eggert_1932_Kongress-Bericht"/>

The DIN system was inspired by [[#Scheiner|Scheiner]]'s system,<ref name="DIN_1934_DIN-4512_Introduction"/> but the sensitivities were represented as the base 10 logarithm of the sensitivity multiplied by 10, similar to [[decibel]]s. Thus an increase of 20° (and not 19° as in Scheiner's system) represented a hundredfold increase in sensitivity, and a difference of 3° was much closer to the base 10 logarithm of 2 (0.30103...):<ref name="Riat_2006"/>

:<math>\log_{10}{(2)} = 0.30103... \approx 3/10</math>.

[[File:Agfacolor neu - 7300960484.jpg|thumb|A box of [[Agfacolor|Agfacolor Neu]] with the instruction "expose as 15/10° DIN" (in German)]]

As in the Scheiner system, speeds were expressed in 'degrees'. Originally the sensitivity was written as a fraction with 'tenths' (for example "18/10°&nbsp;DIN"),<ref name="Benser_1957_Farbig"/> where the resultant value 1.8 represented the relative base 10 logarithm of the speed. 'Tenths' were later abandoned with DIN&nbsp;4512:1957-11, and the example above would be written as "18°&nbsp;DIN".<ref name="Sowerby"/> The degree symbol was finally dropped with DIN&nbsp;4512:1961-10. This revision also saw significant changes in the definition of film speeds in order to accommodate then-recent changes in the American [[#ASA|ASA]] PH2.5-1960 standard, so that film speeds of black-and-white negative film effectively would become doubled, that is, a film previously marked as "18° DIN"<!-- With degree symbol. --> would now be labeled as "21&nbsp;DIN"<!-- Without degree symbol. --> without emulsion changes.

Originally only meant for black-and-white negative film, the system was later extended and regrouped into nine parts, including DIN&nbsp;4512-1:1971-04 for black-and-white negative film, DIN&nbsp;4512-4:1977-06 for color reversal film and DIN&nbsp;4512-5:1977-10 for color negative film.

On an international level the German DIN 4512 system has been effectively superseded in the 1980s by ISO&nbsp;6:1974,<ref name="ISO_6"/> ISO&nbsp;2240:1982,<ref name="http"/> and ISO&nbsp;5800:1979<ref name="iso.org"/> where the same sensitivity is written in linear and logarithmic form as "ISO&nbsp;100/21°" (now again with degree symbol). These [[#ISO|ISO]] standards were subsequently adopted by DIN as well. Finally, the latest DIN&nbsp;4512 revisions were replaced by corresponding ISO standards, DIN&nbsp;4512-1:1993-05 by DIN&nbsp;ISO&nbsp;6:1996-02 in September 2000, DIN&nbsp;4512-4:1985-08 by DIN&nbsp;ISO&nbsp;2240:1998-06 and DIN&nbsp;4512-5:1990-11 by DIN&nbsp;ISO&nbsp;5800:1998-06 both in July 2002.<!-- DIN 4512-3:1973-01 was replaced by DIN 4512-7:1993-01, DIN 4512-8:1993-01 and DIN 4512-9:1993-01. DIN 4512-6:1980-07 was withdrawn. As far as I know, DIN 4512-7:1993-01, DIN 4512-8:1993-01, DIN 4512-9:1993-01 as well as DIN 4512-10:1993-01 are still active as of this writing (2011–07), but don't define film speeds. -->

====BSI{{anchor|BS}}====
When [[British Standards Institution|BS]]&nbsp;935:1941 was published during [[World War II]], specifying exposure tables for negative materials, it employed the same [[#Fixed density|fixed-density]] speed criterion used in the German [[Deutsches Institut für Normung|DIN]]&nbsp;4512:1934 system. The British Standard also used ''logarithmic'' speed numbers, following the example of [[#Scheiner|Scheiner]] and DIN. When the American ASA&nbsp;Z38.2.1:1943 standard was published, it used a [[#Fractional gradient|fractional gradient]] speed criterion and ''arithmetic'' speed numbers, for compatibility with [[Edward Weston|Weston]] and [[GE]].<ref>{{citation
 | last = Jacobson | first = Ralph E
 | publisher = Focal Press 
 | title = The Manual of Photography (Seventh Edition)
 | page = 412
 | year = 1978}}
</ref>

British standard BS&nbsp;1380:1947 adopted the ''fractional gradient'' criterion of the American 1943 standard, and also included ''arithmetic'' speed numbers in addition to ''logarithmic'' numbers.<ref>{{citation
 | last = Jacobson | first = Ralph E
 | publisher = Focal Press 
 | title = The Manual of Photography (Seventh Edition)
 | page = 412
 | year = 1978}}
</ref> The ''logarithmic'' speed number proposed in the later BS&nbsp;1380:1957 standard was almost identical to the DIN&nbsp;4512:1957 standard, except that the BS number was +9 degrees greater than the corresponding DIN number; in 1971, the BS and DIN standards changed this to +10&nbsp;degrees.<ref>{{citation
 | last1 = Dunn | first1 = Jack F
 | last2 = Wakefield | first2 = George L
 | publisher = Fountain Press 
 | title = Exposure Manual (Fourth Edition)
 | page = 22
 | year = 1981}}
</ref>

Following an increasing effort to produce international standards, the British, American, and German standards became identical in ISO&nbsp;6:1974, which corresponded to BS&nbsp;1380:Part1:1973.<ref>{{citation
 | last = Allbright | first = G S
 | journal = The Journal of Photographic Science
 | doi = 10.1080/00223638.1991.11737126
 | title = Emulsion Speed Rating Systems
 | year = 1991| volume = 39
 | issue = 2
 | pages = 95–99
 }}
</ref>

====Weston{{anchor|Weston film speed ratings}}====

[[File:Weston 650 Light Meter July 2015.jpg|thumb|Weston Model 650 [[light meter]] from about 1935]]
[[File:Weston Master Light Meter July 2015.jpg|thumb|Early Weston Master light meter 1935–1945]]

<!-- This section is linked from inside or outside this article, preferable by "#Weston". Do not rename without keeping in sync. -->
Before the advent of the ASA system, the system of ''Weston film speed ratings'' was introduced by [[Edward Faraday Weston]]<!-- No relation with [[Edward Henry Weston]], the photographer. --> (1878–1971) and his father, [[Edward Weston (chemist)|Edward<!-- W.? - see comment below. --> Weston]]<!-- no relation with [[Edward Henry Weston]], the famous photographer --> (1850–1936), a British-born electrical engineer, industrialist and founder of the US-based [[Weston Electrical Instrument Corporation]],<ref name="Mulhern_1990_Letter"/><!-- Note: Both above mentioned patents of Edward F. Weston do not apply to the Weston 617 and the speed rating system. --> with the Weston model 617, one of the earliest photo-electric exposure meters, in August 1932. The meter and film rating system were invented by [[William Nelson Goodwin, Jr.]],<ref name="Goodwin_1938_emulsion_ratings"/><ref name="Roseborough_1996_PHSC-PC-22-3"/> who worked for them<ref name="Tipper_Weston"/><!-- In the Weston Master II model 735 manual (dated 1948?) Weston claimed the Weston film speed rating system was in use for over 15 years already. Reportedly, the Master II was introduced as early as 1945. The manual of the Master Universal model 715 (1939) explicitly mentioned the speed ratings already. The manual of the Universal model 650 (1937) seems to mention them to, but does not name them as such. Goodwin published "Weston emulsion speed ratings" in 1938. So they must have been introduced between 1932 and 1938. --> and later received a [[Howard N. Potts Medal]] for his contributions to engineering.<!-- Franklin Institute, Howard N. Potts Medal, 1958, Engineering, "Contributions to the fields of electrical measuring instruments, and for work on photoelectric exposure meters and establishing a scale for film speeds." http://www.fi.edu/winners/detail.faw?winner_id=2912 --><!-- Goodwin's patents regarding exposure meters: "Photographic exposure meter", US patent 2073790, filed: 1932-10-21, issued 1937-03-16, https://www.google.com/patents/about?id=txdQAAAAEBAJ. "Photoelectric cell circuit", US patent 2032010, filed 1933-06-23, issued 1936-02-25, https://www.google.com/patents/about?id=lixMAAAAEBAJ. "Photoelectric cell circuit", US patent RE20823, filed 1933-06-23, issued 1938-08-09, https://www.google.com/patents/about?id=724CAAAAEBAJ -->

The company tested and frequently published speed ratings for most films of the time. Weston film speed ratings could since be found on most Weston exposure meters and were sometimes referred to by film manufacturers and third parties<ref name="Hefley_1951_micrographs"/> in their exposure guidelines. Since manufacturers were sometimes creative about film speeds, the company went as far as to warn users about unauthorized uses of their film ratings in their "Weston film ratings" booklets.<ref name="Weston_1946_Ratings"/>

The Weston Cadet (model 852 introduced in 1949), Direct Reading (model 853 introduced in 1954) and Master III (models 737 and S141.3 introduced in 1956) were the first in their line of exposure meters to switch and utilize the meanwhile established ASA scale instead. Other models used the original Weston scale up until ca. 1955. The company continued to publish Weston film ratings after 1955,<ref name="Sangamo_1959_Weston-Ratings"/><!-- TBD: Did Weston also publish ratings after the ASA PH2.5-1960 changes? --> but while their recommended values often differed slightly from the ASA film speeds found on film boxes, these newer Weston values were based on the ASA system and had to be converted for use with older Weston meters by subtracting 1/3 exposure stop as per Weston's recommendation.<ref name="Sangamo_1959_Weston-Ratings"/> Vice versa, "old" Weston film speed ratings could be converted into "new" Westons and the ASA scale by adding the same amount, that is, a film rating of 100 Weston (up to 1955) corresponded with 125 ASA (as per ASA PH2.5-1954 and before). This conversion was not necessary on Weston meters manufactured and Weston film ratings published since 1956 due to their inherent use of the ASA system;<!-- temporary sentence follows: --> however, the changes of the ASA PH2.5-1960 revision may be taken into account when comparing with newer ASA or ISO values.<!-- Comparing historical black-and-white negative film speeds given in old Westons with newer ASA values (as per ASA PH2.5-1960) or [[#ISO|ISO]] film speeds may require to add another full exposure stop to the effect of actually adding 1.3 EV to that Weston value in order to find the ISO/ASA equivalent.--><!-- TBD: In order to prove this statement it would be most interesting to learn about Weston speed ratings published after the ASA PH2.5-1960 revision, if they exist. -->

====General Electric{{anchor|GE|General Electric film values}}====
<!-- This section is linked from inside or outside this article, preferable by "#GE". Do not rename without keeping in sync. -->
Prior to the establishment of the ASA scale<ref name="General-Electric_194x_GES-2810_Manual_GW-68"/> and similar to [[#Weston|Weston film speed ratings]] another manufacturer of photo-electric exposure meters, [[General Electric]], developed its own rating system of so-called ''General Electric film values'' (often abbreviated as ''G-E'' or ''GE'') around 1937.<!-- The exposure meter GW-47 is dated 1937 and has GE "film value" scale. -->

Film speed values for use with their meters were published in regularly updated ''General Electric Film Values''<ref name="General-Electric_1947_GED-744_Film-Values"/> leaflets and in the ''General Electric Photo Data Book''.<ref name="General-Electric_1942_GET-I717"/>

General Electric switched to use the [[#ASA|ASA]] scale in 1946. Meters manufactured since February 1946 are equipped with the ASA scale (labeled "Exposure Index") already. For some of the older meters with scales in "Film Speed" or "Film Value" (e.g. models DW-48, DW-49 as well as early DW-58 and GW-68 variants), replaceable hoods with ASA scales were available from the manufacturer.<ref name="General-Electric_1947_GED-744_Film-Values"/><ref name="General-Electric_1946_Ad_Hood"/> The company continued to publish recommended film values after that date, however, they were then aligned to the ASA scale.

====ASA{{anchor|APEX|ASA PH2.5|ASA Z38.2.1}}====
<!-- This section is linked from inside or outside this article, preferable by "#ASA". Do not rename without keeping in sync. -->
<!-- The article Dersu Uzala (1975 film) links to this section. Should the section's title change, fix now mistargeted wikilinks, or use the {{subst:Anchor|}} template to create an anchor to keep the stability of external incoming links too. -->
Based on earlier research work by [[Loyd A. Jones|Loyd Ancile Jones]] (1884–1954) of [[Kodak]]<!-- TBD: Original name of Kodak system was "Kodak speed" since 1939 --> and inspired by the systems of [[#Weston|Weston film speed ratings]]<ref name="Sangamo_1959_Weston-Ratings"/> and [[#GE|General Electric film values]],<ref name="General-Electric_1947_GED-744_Film-Values"/> the [[American Standards Association]] (now named ANSI) defined a new method to determine and specify film speeds of black-and-white negative films in 1943. ASA&nbsp;Z38.2.1–1943 was revised in 1946 and 1947 before the standard grew into ASA PH2.5-1954. Originally, ASA values were frequently referred to as ''American standard speed numbers'' or ''ASA exposure-index numbers''. (See also: [[#EI|Exposure Index]] (EI).)

The ASA scale is a linear scale, that is, a film denoted as having a film speed of 200&nbsp;ASA is twice as fast as a film with 100&nbsp;ASA.

The ASA standard underwent a major revision in 1960 with ASA PH2.5-1960, when the method to determine film speed was refined and previously applied safety factors against under-exposure were abandoned, effectively doubling the nominal speed of many black-and-white negative films. For example, an [[Ilford HP|Ilford HP3]] that had been rated at 200&nbsp;ASA before 1960 was labeled 400&nbsp;ASA afterwards without any change to the emulsion. Similar changes were applied to the [[#DIN|DIN]] system with DIN 4512:1961-10 and the BS system with BS 1380:1963 in the following years.

In addition to the established arithmetic speed scale, ASA PH2.5-1960 also introduced logarithmic ASA grades (100 ASA = 5° ASA), where a difference of 1° ASA represented a full exposure stop and therefore the doubling of a film speed. For some while, ASA grades were also printed on film boxes, and they saw life in the form of the [[APEX system|APEX]] speed value ''S<sub>v</sub>'' (without degree symbol) as well.

ASA PH2.5-1960 was revised as ANSI PH2.5-1979, without the logarithmic speeds, and later replaced by NAPM&nbsp;IT2.5–1986 of the National Association of Photographic Manufacturers, which represented the US adoption of the international standard ISO&nbsp;6. The latest issue of ANSI/NAPM IT2.5 was published in 1993.

The standard for color negative film was introduced as ASA PH2.27-1965 and saw a string of revisions in 1971, 1976, 1979, and 1981, before it finally became ANSI IT2.27–1988 prior to its withdrawal.

Color reversal film speeds were defined in ANSI PH2.21-1983, which was revised in 1989 before it became ANSI/NAPM&nbsp;IT2.21 in 1994, the US adoption of the ISO&nbsp;2240 standard.

On an international level, the ASA system was superseded by the [[#ISO|ISO]] film speed system between 1982 and 1987, however, the arithmetic ASA speed scale continued to live on as the linear speed value of the ISO system.

====GOST{{anchor|GOST 2817|GOST 10691}}====
<!-- This section is linked from inside or outside this article, preferable by "#GOST". Do not rename without keeping in sync. -->
[[File:Svema-film-65.jpg|thumb|A box of [[Svema]] film, with a sensitivity of 65 [[GOST]] (ГОСТ)]]
{{lang|ru-Latn|[[GOST]]|italic=no}} (Cyrillic: {{lang|ru|ГОСТ}}) was an arithmetic film speed scale defined in GOST&nbsp;2817-45 and GOST&nbsp;2817–50.<ref name="Gorokhovskiy_1970_Metrologiya"/><ref name="GOST_2817-50"/> It was used in the former [[Soviet Union]] since October 1951,{{Citation needed|date=August 2011}}<!-- TBD: Why not earlier when GOST 2817-45 was defined earlier? --> replacing Hurter & Driffield (H&D, Cyrillic: ХиД) numbers,<ref name="Gorokhovskiy_1970_Metrologiya"/> which had been used since 1928.{{Citation needed|date=August 2011}}

GOST 2817-50 was similar to the ASA standard, having been based on a speed point at a density 0.2 above base plus fog, as opposed to the ASA's&nbsp;0.1.<ref name="Stroebel_1993_Encyclopedia"/> GOST markings are only found on pre-1987 photographic equipment (film, cameras, [[Light meter|lightmeters]], etc.) of Soviet Union manufacture.<ref name="ZENIT_2011_GOST"/>

On 1 January 1987, the GOST scale was realigned to the [[#ISO|ISO]] scale with GOST&nbsp;10691–84,<ref name="GOST_10691-84"/>

This evolved into multiple parts including GOST&nbsp;10691.6–88<ref name="GOST_10691.6-88"/> and GOST&nbsp;10691.5–88,<ref name="GOST_10691.5-88"/> which both became functional on 1 January 1991.<!-- Also found "GOST 10691-63" and various other GOST 10691.x-xx variants. -->

===Current system: ISO{{anchor|ISO|ISO 6|ISO 2240|ISO 5800|Current ISO system}}===
<!-- This section is linked from inside or outside this article, preferable by "#ISO". Do not rename without keeping in sync. -->
The [[#ASA|ASA]] and [[#DIN|DIN]] film speed standards have been combined into the ISO standards since 1974.

The current [[International standard|International Standard]] for measuring the speed of [[color print film|color negative film]] is ISO&nbsp;5800:2001<ref name="iso.org"/> (first published in 1979, revised in November 1987) from the [[International Organization for Standardization]] (ISO). Related standards ISO&nbsp;6:1993<ref name="ISO_6"/> (first published in 1974) and ISO&nbsp;2240:2003<ref name="http"/> (first published in July 1982, revised in September 1994 and corrected in October 2003) define scales for speeds of black-and-white negative film and color reversal film, respectively.

The determination of ISO speeds [[#ISO 12232|with digital still-cameras]] is described in ISO&nbsp;12232:2019 (first published in August 1998, revised in April 2006, corrected in October 2006 and again revised in February 2019).<ref name="ISO 12232_2019"/><ref name="Gasiorowski-Denis_2019_ISO12232"/>

The ISO system defines both an [[arithmetic]] and a [[logarithmic scale]].<ref name="manual"/> The arithmetic ISO scale corresponds to the arithmetic ASA system, where a doubling of film sensitivity is represented by a doubling of the numerical film speed value. In the logarithmic ISO scale, which corresponds to the DIN scale, adding 3° to the numerical value constitutes a doubling of sensitivity. For example, a film rated ISO&nbsp;200/24° is twice as sensitive as one rated ISO&nbsp;100/21°.<ref name="manual"/>

Commonly, the logarithmic speed is omitted; for example, "ISO&nbsp;100" denotes "ISO&nbsp;100/21°",<ref name="Graves_1996_Zone-system"/> while logarithmic ISO speeds are written as "ISO&nbsp;21°" as per the standard.

{{Sequence
| prev = [[ISO 5]]
| list = Lists of ISOs
| curr = ISO 6
| next = [[ISO 7]]
}}

===Conversion between current scales{{anchor|Conversion}}===
<!-- This section is linked from inside or outside this article, preferable by "#Conversion". Do not rename without keeping in sync. -->
[[File:ASA and DIN on Yashica FR.jpg|right|thumb|A [[Yashica]] FR with both ASA and DIN markings]]
Conversion from arithmetic speed ''S'' to logarithmic speed ''S''° is given by<ref name="ISO_6"/>

:<math>S^\circ = 10 \log S + 1</math>

and rounding to the nearest integer; the log is base 10. Conversion from logarithmic speed to arithmetic speed is given by<ref name="ISO-2721_1982"/>

:<math>S = 10^{\left( {S^\circ - 1} \right)/10}</math>

and rounding to the nearest standard arithmetic speed in Table&nbsp;1 below.
{{table alignment}}{{mw-datatable}}
{| class="wikitable mw-datatable defaultcenter col-1left" summary="Comparison of speed scales APEX S<sub>v</sub>, ISO (since 1974), digital cameras, ASA (since 1960), DIN (since 1961) and GOST (1951 to 1986)."
|+ Table1. Comparison of various film speed scales
! style="max-width:4em" | [[#APEX|APEX]] ''S<sub>v</sub>''
! [[#ISO|ISO]]{{br}}{{abbr|arith|arithmetic}}/{{wbr}}{{abbr|log|logarithmic}}°
! style="max-width:4em" | Camera mfrs.
! [[#ASA|ASA]]{{br}}{{abbr|arith|arithmetic}}
! [[#DIN|DIN]]{{br}}{{abbr|log|logarithmic}}
! [[#GOST|GOST]]{{br}}{{abbr|arith|arithmetic}}
! Example of film stock with this nominal speed
|- style="background:#ffe"
| −2
| 0.8/0°{{wbr}}<ref name="Leica_2002_ISO_R9"/>
|
| 0.8
| 0<ref name="Leica_1996_ISO_R8"/>
|
| FPPBW Super Positive<ref>{{Cite web |title=35mm BW Film - FPP Super Positive Film (1 Roll) |url=https://filmphotographystore.com/products/35mm-bw-film-svema-super-positive-film-1-roll |access-date=2023-05-10 |website=Film Photography Project Store}}</ref>
|-
|
| 1/1°
|
| 1
| 1
| (1)
| Svema Micrat-orto, Astrum Micrat-orto
|-
|
| 1.2/2°
|
| 1.2
| 2
| (1)
|
|- style="background:#ffe"
| −1
| 1.6/3°
|
| 1.6
| 3
| 1.4
|
|-
|
| 2/4°
|
| 2
| 4
| (2)
|
|-
|
| 2.5/5°
|
| 2.5
| 5
| (2)
|
|- style="background:#ffe"
| 0
| 3/6°
|
| 3
| 6
| 2.8
| Svema MZ-3, Astrum MZ-3
|-
|
| '''4/7°'''
|
| '''4'''
| 7
| (4)
|
|-
|
| '''5/8°'''
|
| '''5'''
| 8
| (4)
| Original three-strip [[Technicolor#Convincing Hollywood|Technicolor]]
|- style="background:#ffe"
| '''1'''
| '''6/9°'''
|
| '''6'''
| 9
| 5.5
| Original [[Kodachrome]]
|-
|
| '''8/10°'''
|
| '''8'''
| 10
| (8)
| Polaroid PolaBlue
|-
|
| '''10/11°'''
|
| '''10'''
| 11
| (8)
| [[Kodachrome|Kodachrome 8mm film]]
|- style="background:#ffe"
| '''2'''
| '''12/12°'''
|
| '''12'''
| 12
| 11
| Gevacolor 8mm reversal film, later Agfa Dia-Direct
|-
|
| '''16/13°'''
|
| '''16'''
| 13
| (16)
| Agfacolor 8mm reversal film
|-
|
| '''20/14°'''
|
| '''20'''
| 14
| (16)
| Adox CMS 20
|- style="background:#ffe"
| '''3'''
| '''25/15°'''
|
| '''25'''
| 15
| 22
| Old Agfacolor, KodachromeII and (later) [[Kodachrome|Kodachrome 25]], Efke 25
|-
|
| '''32/16°'''
|
| '''32'''
| 16
| (32)
| Kodak Panatomic-X
|-
|
| '''40/17°'''
|
| '''40'''
| 17
| (32)
| [[Kodachrome|Kodachrome 40 (movie)]]
|- style="background:#ffe"
| '''4'''
| '''50/18°'''
|
| '''50'''
| 18
| 45
| Fuji RVP ([[Velvia]]), Ilford Pan F Plus, Kodak Vision 250D 5201 (movie), AGFA CT18, Efke 50, Polaroid type 55
|-
|
| '''64/19°'''
|
| '''64'''
| 19
| (65)
| [[Kodachrome|Kodachrome 64]], Ektachrome-X, Polaroid type 64T
|-
|
| '''80/20°'''
|
| '''80'''
| 20
| (65)
| Ilford Commercial Ortho, Polaroid type669
|- style="background:#ffe"
| '''5'''
| '''100/21°'''
|
| '''100'''
| 21
| 90
| [[Kodak T-MAX]] 100 (TMX), Kodak [[Ektar]], [[Fujichrome Provia 100F]], [[Neopan|Fujifilm Acros 100 II]] Efke 100, Fomapan/Arista 100, Kentmere Pan 100
|-
|
| '''125/22°'''
|
| '''125'''
| 22
| (130)
| [[Ilford FP4+]], Kodak Plus-X Pan, Svema Color 125
|-
|
| '''160/23°'''
|
| '''160'''
| 23
| (130)
| [[Fujicolor Pro|Fujicolor Pro 160C/S]], Kodak High-Speed Ektachrome, [[Kodak Portra]] 160NC and 160VC
|- style="background:#ffe"
| '''6'''
| '''200/24°'''
|
| '''200'''
| 24
| 180
| [[Kodacolor (still photography)|Kodak Gold 200]], [[Fujifilm Superia|Fujicolor Superia]] 200, [[Agfa Scala 200x]], Fomapan/Arista 200, Wittner Chrome 200D, Agfa Aviphot Chrome 200 PE1
|-
|
| '''250/25°'''
|
| '''250'''
| 25
| (250)
| Tasma Foto-250, Eastman Double-X
|-
|
| '''320/26°'''
|
| '''320'''
| 26
| (250)
| Kodak Tri-X Pan Professional (TXP)
|- style="background:#ffe"
| '''7'''
| '''400/27°'''
|
| '''400'''
| 27
| 350
| Kodak T-Max400 (TMY), [[Kodak Tri-X 400]], [[Kodak Portra]] 400, [[Ilford HP5+]], [[Fujifilm Superia X-tra 400]], [[Fujichrome Provia 400X]], Fomapan/Arista 400, KentmerePan 400
|-
|
| '''500/28°'''
|
| '''500'''
| 28
| (500)
| Kodak Vision3500T5219 (movie)
|-
|
| '''640/29°'''
|
| '''640'''
| 29
| (500)<!-- (560) -->
| Polaroid600
|- style="background:#ffe"
| '''8'''
| '''800/30°'''
|
| '''800'''
| 30
| 700
| [[Fujicolor Pro|Fuji Pro 800Z]], [[Instax|Fuji Instax]]
|-
|
| '''1000/31°'''
|
| '''1000'''
| 31
| (1000)
| [[Ilford Delta]] 3200, [[Kodak T-MAX|Kodak P3200 TMAX]]<ref name="TMZ"/><br/>Kodak Professional T-Max P3200<ref name="new TMZ"/> <small>(see [[#Marketing anomalies|Marketing anomalies]] below)</small>
|-
|
| '''1250/32°'''
|
| '''1250'''
| 32
| (1000)
| Kodak Royal-X Panchromatic
|- style="background:#ffe"
| '''9'''
| '''1600/33°'''
|
| '''1600'''
| 33
| 1400 (1440)
| Fujicolor 1600, [[Fujifilm_Superia|Fuji Natura 1600 and Superia 1600]], [[Neopan#Neopan 1600 Professional (Super Presto)|Neopan 1600]]
|-
|
| '''2000/34°'''
|
| '''2000'''
| 34
| (2000)
|
|-
|
| '''2500/35°'''
|
| '''2500'''
| 35
| (2000)
|
<!--
|-
|
|
| 3000
| ~36
|
| Polaroid Black and White 'Peel apart' Film
-->
|- style="background:#ffe"
| '''10'''
| '''3200/36°'''
|
| '''3200'''
| 36
| 2800 (2880)
| Konica 3200, Polaroid type 667, Fujifilm FP-3000B, Kodak Tmax 3200 B&W{{Note|Tmax 3200 is not actually rated to ISO standards at 3200, but delivers useful results when exposed at 3200.}}
|-
|
| '''4000/37°'''
|
|
| 37
| (4000)
|
|-
|
| '''5000/38°'''
|
|
| 38
| (4000)
|
|- style="background:#ffe"
| 11
| '''6400/39°'''
|
| 6400{{wbr}}<ref name="ASA_1961_PH2-12_12500"/>
| 39
| 5600
|
|-
|
| '''8000/40°'''{{wbr}}<ref name="Leica_2002_ISO_R9"/><ref name="Leica_1996_ISO_R8"/>
|
|
|
|
|
|-
|
| '''10000/41°'''{{wbr}}<ref name="Leica_2002_ISO_R9"/><ref name="Leica_1996_ISO_R8"/><ref name="Vision_2016_Boosting"/>
|
|
|
|
|
|- style="background:#ffe"
| 12
| 12500/42°{{wbr}}<ref name="Leica_2002_ISO_R9"/><ref name="Vision_2016_Boosting"/>
| 12800{{wbr}}<ref name="Leica_1996_ISO_R8"/><ref name="Canon_1966_Booster"/><ref name="Canon_1978_Manual_A-1"/><ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
| 12500{{wbr}}<ref name="ASA_1961_PH2-12_12500"/>
|
|
| ISO speeds greater than 10000 have not been defined officially before ISO12232:2019.<ref name="ISO 12232_2019"/>
|-
|
| 16000/43°{{wbr}}<ref name="Vision_2016_Boosting"/>
|
|
|
|
|
|-
|
| 20000/44°{{wbr}}<ref name="Vision_2016_Boosting"/>
|
|
|
|
| Polaroid type 612<ref>{{cite news |last1=Manning |first1=Jack |title=CAMERA; NEW DEVELOPMENTS IN INSTANT PHOTOGRAPHY |work=The New York Times |url=https://www.nytimes.com/1983/11/27/arts/camera-new-developments-in-instant-photography.html |access-date=5 February 2024 |agency=New York Times |date=November 27, 1983 |page=36}}</ref>
|-
|- style="background:#ffe"
| 13
| 25000/45°{{wbr}}<ref name="Vision_2016_Boosting"/>
| 25600{{wbr}}<ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
|
|
|
|
|-
|
| 32000/46°{{wbr}}<ref name="Vision_2016_Boosting"/>
|
|
|
|
|
|-
|
| 40000/47°{{wbr}}<ref name="Vision_2016_Boosting"/>
|
|
|
|
|
|- style="background:#ffe"
| 14
| 50000/48°{{wbr}}<ref name="Vision_2016_Boosting"/>
| 51200{{wbr}}<ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
|
|
|
|
|-
|
| 64000/49°{{wbr}}<ref name="Vision_2016_Boosting"/>
|
|
|
|
|
|-
|
| 80000/50°{{wbr}}<ref name="Vision_2016_Boosting"/>
|
|
|
|
|
|- style="background:#ffe"
| 15
| 100000{{wbr}}/51°{{wbr}}<ref name="Leica_2002_ISO_R9"/>
| 102400{{wbr}}<ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
|
| 51<ref name="Leica_1996_ISO_R8"/>
|
| [[Nikon D3s]] and [[Canon EOS-1D Mark IV]] (2009)
|-
|
| 125000{{wbr}}/52°
|
|
|
|
|
|-
|
| 160000{{wbr}}/53°
|
|
|
|
|
|- style="background:#ffe"
| 16
| 200000{{wbr}}/54°
| 204800{{wbr}}<ref name="Canon_2011_ISO_EOS-1D_X"/><ref name="Nikon_2012_ISO_D4"/><ref name="Ricoh_2014_ISO_Pentax_645Z"/>
|
|
|
| [[Canon EOS-1D X]] (2011), [[Nikon D4]] (2012), [[Pentax 645Z]] (2014)
|-
|
| 250000{{wbr}}/55°
|
|
|
|
|
|-
|
| 320000{{wbr}}/56°
|
|
|
|
|
|- style="background:#ffe"
| 17
| 400000{{wbr}}/57°
| 409600{{wbr}}<ref name="Nikon_2014_ISO_D4s"/><ref name="Sony_2014_ISO_Alpha_ILCE-7S"/>
|
|
|
| [[Nikon D4s]], [[Sony α ILCE-7S]] (2014), [[Canon EOS 1D X Mark II]] (2016)
|-
|
| 500000{{wbr}}/58°
|
|
|
|
|
|-
|
| 640000{{wbr}}/59°
|
|
|
|
|
|- style="background:#ffe"
| 18
| 800000{{wbr}}/60°
|
|
|
|
|
|-
|
| 1000000{{wbr}}/61°
|
|
|
|
|
|-
|
| 1250000{{wbr}}/62°
|
|
|
|
|
|- style="background:#ffe"
| 19
| 1600000{{wbr}}/63°
|
|
|
|
|
|-
|
| 2000000{{wbr}}/64°
|
|
|
|
|
|-
|
| 2500000{{wbr}}/65°
|
|
|
|
|
|- style="background:#ffe"
| 20
| 3200000{{wbr}}/66°
| 3280000
|
|
|
| [[Nikon D5]] (2016)
|-
|
| 4000000{{wbr}}/67°{{wbr}}<ref name="Canon_2015_ISO_ME20F-SH"/>
| 4560000
|
|
|
| [[Canon ME20F-SH]]<ref name="Canon_2015_ISO_ME20F-SH"/> (2015)
|}

Table notes:
# Speeds shown in bold under APEX, ISO, and ASA are values actually assigned in speed standards from the respective agencies; other values are calculated extensions to assigned speeds using the same progressions as for the assigned speeds.
# APEX&nbsp;''S<sub>v</sub>'' values 1 to 10 correspond with logarithmic ASA grades 1° to 10° found in ASA&nbsp;PH2.5-1960.
# ASA arithmetic speeds from 4 to 5 are taken from ANSI&nbsp;PH2.21-1979 (Table&nbsp;1, p.&nbsp;8).
# ASA arithmetic speeds from 6 to 3200 are taken from ANSI PH2.5-1979 (Table&nbsp;1, p.&nbsp;5) and ANSI&nbsp;PH2.27-1979.
# ISO arithmetic speeds from 4 to 3200 are taken from ISO&nbsp;5800:1987 (Table "ISO speed scales", p.&nbsp;4).
# ISO arithmetic speeds from 6 to 10000 are taken from ISO&nbsp;12232:1998 (Table&nbsp;1, p.&nbsp;9).
# ISO&nbsp;12232:1998 does not specify speeds greater than 10000. However, the upper limit for ''S''<sub>noise</sub>&nbsp;10000 was given as 12500, suggesting that ISO may have envisioned a progression of 12500, 25000, 50000, and 100000, similar to that from 1250 to 10000. This was consistent with ASA PH2.12-1961.<ref name="ASA_1961_PH2-12_12500"/> For digital cameras, Nikon, Canon, Sony, Pentax, and Fujifilm chose to express the greater speeds in an exact power-of-2 progression from the highest previously realized speed (6400) rather than rounding to an extension of the existing progression. Speed ratings greater than 10000 have finally been defined in ISO&nbsp;12232:2019.<ref name="ISO 12232_2019"/>
# Most of the modern [[35mm format|35 mm film]] SLRs support an automatic film speed range from ISO&nbsp;25/15° to 5000/38° with [[DX encoding|DX-coded films]], or ISO&nbsp;6/9° to 6400/39° manually (without utilizing [[exposure compensation]]). The film speed range with support for [[Through-the-lens metering#Through the lens flash metering|TTL flash]] is smaller, typically ISO&nbsp;12/12° to 3200/36° or less.
# The Booster<ref name="Canon_1966_Booster"/> accessory for the [[Canon Pellix QL]] (1965) and [[Canon FT QL]] (1966) supported film speeds from 25 to 12800&nbsp;ASA.
# The film speed dial of the [[Canon A-1]] (1978) supported a speed range from 6 to 12800 ASA (but already called ISO film speeds in the manual).<ref name="Canon_1978_Manual_A-1"/> On this camera exposure compensation and extreme film speeds were mutually exclusive.
# The [[Leica R8|Leica&nbsp;R8]] (1996) and [[Leica R9|R9]] (2002) officially supported film speeds of 8000/40°, 10000/41° and 12800/42° (in the case of the R8) or 12500/42° (in the case of the R9), and utilizing its ±3 EV exposure compensation the range could be extended from ISO&nbsp;0.8/0° to ISO&nbsp;100000/51° in half exposure steps.<ref name="Leica_2002_ISO_R9"/><ref name="Leica_1996_ISO_R8"/>
# Digital camera manufacturers' arithmetic speeds from 12800 to 409600 are from specifications by Nikon (12800, 25600, 51200, 102400 in 2009,<ref name="Nikon_2009_ISO_D3s"/> 204800 in 2012,<ref name="Nikon_2012_ISO_D4"/> 409600 in 2014<ref name="Nikon_2014_ISO_D4s"/>), Canon (12800, 25600, 51200, 102400 in 2009,<ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/> 204800 in 2011,<ref name="Canon_2011_ISO_EOS-1D_X"/> 4000000 in 2015<ref name="Canon_2015_ISO_ME20F-SH"/>), Sony (12800 in 2009,<ref name="Sony_2009_ISO_Alpha_DSLR-A500-A550"/> 25600 in 2010,<ref name="Sony_2010_ISO_Alpha_DSLR-A560-A580"/> 409600 in 2014<ref name="Sony_2014_ISO_Alpha_ILCE-7S"/>), Pentax (12800, 25600, 51200 in 2010,<ref name="Pentax_2010_ISO_K-5"/> 102400, 204800 in 2014<ref name="Ricoh_2014_ISO_Pentax_645Z"/>), and Fujifilm (12800 in 2011<ref name="Fujifilm_2011_ISO_X100"/>).

=== Historic ASA and DIN conversion ===
[[File:ASA DIN conversion table.jpg|thumb|300px|Historic film speed conversion table, 1952<ref name="Singapore_1952"/>]]
[[File:Tessina with exposure guide plate.JPG|thumb|300px|Classic camera [[Tessina]] with exposure guide, late 1950s]]

As discussed in the ASA and DIN sections, the definition of the ASA and DIN scales changed several times in the 1950s up into the early 1960s making it necessary to convert between the different scales. Since the ISO system combines the newer ASA and DIN definitions, this conversion is also necessary when comparing older ASA and DIN scales with the ISO scale.

The picture shows an ASA/DIN conversion in a 1952 photography book<ref name="Singapore_1952"/> in which 21/10°&nbsp;DIN<!-- This convention used up to 1957. --> was converted to ASA&nbsp;80 instead of ASA&nbsp;100.

Some classic camera's exposure guides show the old conversion as they were valid at the time of production, for example the exposure guide of the classic camera [[Tessina]] (since 1957), where 21/10°&nbsp;DIN is related to ASA&nbsp;80, 18°&nbsp;DIN to ASA&nbsp;40, etc. Users of classic cameras may become confused if they are not aware of the historic background of changing standards.

===Determining film speed===
[[File:ISO6speedMethod.png|thumb|540px|ISO&nbsp;6:1993 method of determining speed for black-and-white film.]]
[[File:Amsterdam, Red Ligth District, Graffiti 1996.jpg|thumb|200px|Recording film 1000 ASA, Red Light District, Amsterdam, Graffiti 1996]]

Film speed is found from a plot of [[optical density]] vs. log of exposure for the film, known as the ''D''–log ''H'' curve or [[sensitometry|Hurter–Driffield]] curve. There typically are five regions in the curve: the base + fog, the toe, the linear region, the shoulder, and the overexposed region. For [[black-and-white]] [[negative film]], the "speed point" m is the point on the curve where density exceeds the base + fog density by 0.1 when the negative is developed so that a point n where the log of exposure is 1.3 units greater than the exposure at point m has a density 0.8 greater than the density at point m. The exposure ''H''<sub style="font-size: 70%'">m</sub>, in [[Lux second|lux-s]], is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed is determined from:

:<math>S = \frac {0.8\;\text{lx⋅s}} {H_\mathrm{m}}</math>

This value is then rounded to the nearest standard speed in Table&nbsp;1 of ISO&nbsp;6:1993.

Determining speed for color negative film is similar in concept but more complex because it involves separate curves for blue, green, and red. The film is processed according to the film manufacturer's recommendations rather than to a specified contrast. ISO speed for [[color reversal film]] is determined from the middle rather than the threshold of the curve; it again involves separate curves for blue, green, and red, and the film is processed according to the film manufacturer's recommendations.

===Applying film speed===
Film speed is used in the [[Light meter#Exposure equations|exposure equations]] to find the appropriate exposure parameters. Four variables are available to the photographer to obtain the desired effect: [[lighting]], film speed, [[f-number]] (aperture size), and [[shutter speed]] (exposure time). The equation may be expressed as ratios, or, by taking the logarithm (base 2) of both sides, by addition, using the APEX system, in which every increment of 1 is a doubling of exposure; this increment is commonly known as a "stop". The [[F-number#Working f-number|effective f-number]] is proportional to the ratio between the lens [[focal length]] and [[aperture]] diameter, the diameter itself being proportional to the square root of the aperture area. Thus, a lens set to {{f/|1.4|link=yes}} allows twice as much light to strike the focal plane as a lens set to {{f/}}2. Therefore, each f-number factor of the square root of two (approximately 1.4) is also a stop, so lenses are typically marked in that progression: {{f/}}1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, etc..

The ISO arithmetic speed has a useful property for photographers without the equipment for taking a metered light reading. Correct exposure will usually be achieved for a frontlighted scene in bright sun if the aperture of the lens is set to f/16 and the shutter speed is the reciprocal of the ISO film speed (e.g. 1/100 second for 100&nbsp;ISO film). This known as the [[sunny 16 rule]].

===Exposure index{{anchor|EI}}===
<!-- This section is linked from inside or outside this article, preferable by "#EI". Do not rename without keeping in sync. -->
Exposure index, or EI, refers to speed rating assigned to a particular film and shooting situation in variance to the film's actual speed. It is used to compensate for equipment calibration inaccuracies or process variables, or to achieve certain effects. The exposure index may simply be called the ''speed setting'', as compared to the speed ''rating''.

For example, a photographer may rate an ISO&nbsp;400 film at EI&nbsp;800 and then use [[push processing]] to obtain printable negatives in low-light conditions. The film has been exposed at EI&nbsp;800.

Another example occurs where a camera's [[shutter (photography)|shutter]] is miscalibrated and consistently overexposes or underexposes the film; similarly, a [[light meter]] may be inaccurate. One may adjust the EI setting accordingly in order to compensate for these defects and consistently produce correctly exposed negatives.

==Reciprocity==
Upon exposure, the amount of light energy that reaches the film determines the effect upon the emulsion. If the brightness of the light is multiplied by a factor and the exposure of the film decreased by the same factor by varying the camera's [[shutter speed]] and aperture, so that the energy received is the same, the film will be developed to the same density. This rule is called [[reciprocity (photography)|reciprocity]]. The systems for determining the sensitivity for an emulsion are possible because reciprocity holds over a wide range of customary conditions. In practice, reciprocity works reasonably well for normal photographic films for the range of exposures between 1/1000&nbsp;second to 1/2&nbsp;second. However, this relationship breaks down outside these limits, a phenomenon known as [[Reciprocity (photography)#Reciprocity failure|reciprocity failure]].<ref name="Lambrecht_2003_Way-Beyond"/>

==Film sensitivity and grain==
{{Main|Film grain}}
[[File:Korn HP4 ISO1600.jpg|thumb|Grainy high-speed B&W film negative]]
The size of [[silver halide]] grains in the [[emulsion]] affects film sensitivity, which is related to [[film grain|granularity]] because larger grains give film greater sensitivity to light. Fine-grain film, such as film designed for portraiture or copying [[original camera negative]]s, is relatively insensitive, or "slow", because it requires brighter light or a longer exposure than a "fast" film. Fast films, used for photographing in low light or capturing high-speed motion, produce comparatively grainy images.

[[Eastman Kodak|Kodak]] has defined a "Print Grain Index" (PGI) to characterize film grain (color negative films only), based on perceptual [[just-noticeable difference]] of graininess in prints. They also define "granularity", a measurement of grain using an RMS measurement of density fluctuations in uniformly exposed film, measured with a microdensitometer with 48 micrometre aperture.<ref name="Kodak_2000_Print-Grain-Index"/> Granularity varies with exposure&nbsp;— underexposed film looks grainier than overexposed film.

===Marketing anomalies===
<!-- This section is linked from several places inside this article. Do not remove or rename this section heading without also updating the links. -->
Some high-speed black-and-white films, such as [[Ilford Delta]]&nbsp;3200, [[Kodak T-MAX|P3200&nbsp;T-Max]], and T-MAX&nbsp;P3200 are marketed with film speeds in excess of their true ISO speed as determined using the ISO testing method. According to the respective data sheets, the Ilford product is actually an ISO&nbsp;1000 film,<ref name="Ilford_2010_Delta3200"/> while the Kodak film's speed is nominally 800 to 1000&nbsp;ISO.<ref name="TMZ"/><ref name="new TMZ"/> The manufacturers do not indicate that the 3200 number is an ISO rating on their packaging.<ref name="Ilford_2006_Delta-3200"/> Kodak and Fuji also marketed E6 films designed for pushing (hence the "P" prefix), such as Ektachrome&nbsp;P800/1600 and Fujichrome&nbsp;P1600, both with a base speed of ISO&nbsp;400. The [[DX encoding|DX codes]] on the film cartridges indicate the marketed film speed (i.e. 3200), not the ISO speed, in order to automate shooting and development.

==Digital camera ISO speed and exposure index{{anchor|Digital|ISO speed}}==
{{Unreferenced section|date=October 2016}}
<!-- This section is linked from inside or outside this article, preferable by "#Digital". Do not rename without keeping in sync. -->
[[File:CCD Image sensor.jpg|thumb|300px|A [[Charge-coupled device|CCD]] image sensor, 2/3 inch size]]
In [[Digital camera|digital camera systems]], an arbitrary relationship between exposure and sensor data values can be achieved by setting the [[signal gain]] of the sensor. The relationship between the sensor data values and the lightness of the finished image is also arbitrary, depending on the parameters chosen for the interpretation of the sensor data into an image [[color space]] such as [[sRGB]].

For digital photo cameras ("digital still cameras"), an [[#EI|exposure index]] (EI) rating—commonly called ''ISO'' setting—is specified by the manufacturer such that the sRGB image files produced by the camera will have a lightness similar to what would be obtained with film of the same EI rating at the same exposure. The usual design is that the camera's parameters for interpreting the sensor data values into sRGB values are fixed, and a number of different EI choices are accommodated by varying the sensor's signal gain in the analog realm, prior to conversion to digital. Some camera designs provide at least some EI choices by adjusting the sensor's signal gain in the digital realm ("expanded ISO"). A few camera designs also provide EI adjustment through a choice of lightness parameters for the interpretation of sensor data values into sRGB; this variation allows different tradeoffs between the range of highlights that can be captured and the amount of noise introduced into the shadow areas of the photo.

Digital cameras have far surpassed film in terms of sensitivity to light, with [[#ISO|ISO]] equivalent speeds of up to 4,560,000, a number that is unfathomable in the realm of conventional film photography. Faster [[microprocessor]]s, as well as advances in software noise reduction techniques allow this type of processing to be executed the moment the photo is captured, allowing photographers to store images that have a higher level of refinement and would have been prohibitively time-consuming to process with earlier generations of digital camera hardware.

===The ISO (International Organization of Standards) 12232:2019 standard{{anchor|ISO 12232|REI|SOS}}===

<!-- This section is linked from inside or outside this article, preferable by "#ISO 12232". Do not rename without keeping in sync. -->
The ISO standard ISO&nbsp;12232:2006<ref name="iso12232"/> gave digital still camera manufacturers a choice of five different techniques for determining the exposure index rating at each sensitivity setting provided by a particular camera model. Three of the techniques in ISO&nbsp;12232:2006 were carried over from the 1998 version of the standard, while two new techniques allowing for measurement of JPEG output files were introduced from [[Camera & Imaging Products Association|CIPA]] DC-004.<ref name="CIPA_DC-004"/> Depending on the technique selected, the exposure index rating could depend on the sensor sensitivity, the sensor noise, and the appearance of the resulting image. The standard specified the measurement of light sensitivity of the entire digital camera system and not of individual components such as digital sensors, although Kodak has reported<ref name="Kodak_Sensor_ISO"/> using a variation to characterize the sensitivity of two of their sensors in 2001.

The ''Recommended Exposure Index'' (REI) technique, new in the 2006 version of the standard, allows the manufacturer to specify a camera model's [[#EI|EI]] choices arbitrarily. The choices are based solely on the manufacturer's opinion of what EI values produce well-exposed [[sRGB]] images at the various sensor sensitivity settings. This is the only technique available under the standard for output formats that are not in the sRGB color space. This is also the only technique available under the standard when [[Metering mode#Multi-zone metering|multi-zone metering]] (also called ''pattern'' metering) is used.

The ''Standard Output Sensitivity'' (SOS) technique, also new in the 2006 version of the standard, effectively specifies that the average level in the sRGB image must be 18% gray plus or minus 1/3 stop when the exposure is controlled by an automatic exposure control system calibrated per [[Light Meter|ISO&nbsp;2721]] and set to the EI with no [[exposure compensation]]. Because the output level is measured in the sRGB output from the camera, it is only applicable to sRGB {{nowrap|images—typically}} [[JPEG]]—and not to output files in [[raw image format]]. It is not applicable when multi-zone metering is used.

The CIPA DC-004 standard requires that Japanese manufacturers of digital still cameras use either the REI or SOS techniques, and DC-008<ref name="CIPA_DC-008"/> updates the [[Exchangeable image file format|Exif]] specification to differentiate between these values. Consequently, the three EI techniques carried over from ISO&nbsp;12232:1998 are not widely used in recent camera models (approximately 2007 and later). As those earlier techniques did not allow for measurement from images produced with [[lossy compression]], they cannot be used at all on cameras that produce images only in JPEG format.

The ''saturation-based'' (SAT or S<sub>sat</sub>) technique is closely related to the SOS technique, with the sRGB output level being measured at 100% white rather than 18% gray. The SOS value is effectively 0.704 times the saturation-based value.<ref name="Kerr_2007_Digital-Camera-Sensitivity"/> Because the output level is measured in the [[sRGB]] output from the camera, it is only applicable to sRGB images—typically [[Tagged Image File Format|TIFF]]—and not to output files in raw image format.{{citation needed|date=August 2019}} It is not applicable when multi-zone metering is used.

The two ''noise-based'' techniques have rarely been used for consumer digital still cameras.{{citation needed|date=August 2019}} These techniques specify the highest EI that can be used while still providing either an "excellent" picture or a "usable" picture depending on the technique chosen.{{citation needed|date=August 2019}}

An update to this standard has been published as ISO&nbsp;12232:2019, defining a wider range of ISO speeds.<ref name="ISO 12232_2019"/><ref name="Gasiorowski-Denis_2019_ISO12232"/>

===Measurements and calculations===
ISO speed ratings of a digital camera are based on the properties of the sensor and the image processing done in the camera, and are expressed in terms of the luminous exposure ''H'' (in [[lux]] [[second]]s) arriving at the sensor. For a typical camera lens with an effective [[focal length]] ''f'' that is much smaller than the distance between the camera and the photographed scene, ''H'' is given by
:<math>H=\frac{qLt}{N^2}</math>,
where ''L'' is the [[luminance]] of the scene (in [[candela]] per [[metre|m]]²), ''t'' is the exposure time (in seconds), ''N'' is the aperture f-number, and
:<math>q=\frac{\pi}{4} T\,v(\theta)\,\cos^4\theta</math>
is a factor depending on the [[transmittance]] ''T'' of the lens, the [[vignetting]] factor ''v''(''θ''), and the angle ''θ'' relative to the axis of the lens. A typical value is ''q''&nbsp;=&nbsp;0.65, based on ''θ''&nbsp;=&nbsp;10°, ''T''&nbsp;=&nbsp;0.9, and ''v''&nbsp;=&nbsp;0.98.<ref name="ISO-12232_1998"/>

====Saturation-based speed====
The ''saturation-based speed'' is defined as
:<math>S_{\mathrm{sat}}=\frac{78\;\text{lx⋅s}}{H_{\mathrm{sat}}}</math>,
where <math>H_{\mathrm{sat}}</math> is the maximum possible exposure that does not lead to a clipped or bloomed camera output. Typically, the lower limit of the saturation speed is determined by the sensor itself, but with the [[Gain (electronics)|gain]] of the amplifier between the sensor and the [[analog-to-digital converter]], the saturation speed can be increased. The factor 78 is chosen such that exposure settings based on a standard [[light meter]] and an 18-percent reflective surface will result in an image with a grey level of 18%/{{radic|2}}&nbsp;=&nbsp;12.7% of saturation. The factor {{radic|2}} indicates that there is half a stop of headroom to deal with [[specular reflection]]s that would appear brighter than a 100% reflecting diffuse white surface.<ref name="iso12232"/>

====Noise-based speed====
{{main|Signal to noise ratio (imaging)}}[[File:ISO.gif|thumb|Digital noise at 3200 ISO vs. 100 ISO]]
The ''noise-based speed'' is defined as the exposure that will lead to a given [[signal-to-noise ratio]] on individual [[pixel]]s. Two ratios are used, the 40:1 ("excellent image quality") and the 10:1 ("acceptable image quality") ratio. These ratios have been subjectively determined based on a resolution of 70&nbsp;pixels per cm (178 DPI) when viewed at 25&nbsp;cm (9.8&nbsp;inch) distance. The noise is defined as the [[standard deviation]] of a weighted average of the [[luminance]] and color of individual pixels. The noise-based speed is mostly determined by the properties of the sensor and somewhat affected by the noise in the electronic gain and AD converter.<ref name="iso12232"/>

====Standard output sensitivity (SOS){{anchor|SOS}}====
<!-- This section is linked from inside or outside this article, preferable by "#SOS". Do not rename without keeping in sync. -->
In addition to the above speed ratings, the standard also defines the ''standard output sensitivity'' (SOS), how the exposure is related to the digital pixel values in the output image. It is defined as
:<math>S_{\mathrm{sos}} = \frac{10\;\text{lx⋅s}}{H_{\mathrm{sos}}},</math>
where <math>H_{\mathrm{sos}}</math> is the exposure that will lead to values of 118 in 8-bit pixels, which is 18 percent of the saturation value in images encoded as [[sRGB]] or with [[gamma correction|gamma]]&nbsp;=&nbsp;2.2.<ref name="iso12232"/>

====Discussion====
The standard specifies how speed ratings should be reported by the camera. If the noise-based speed (40:1) is ''higher'' than the saturation-based speed, the noise-based speed should be reported, rounded ''downwards'' to a standard value (e.g. 200, 250, 320, or 400). The rationale is that exposure according to the lower saturation-based speed would not result in a visibly better image. In addition, an exposure latitude can be specified, ranging from the saturation-based speed to the 10:1 noise-based speed. If the noise-based speed (40:1) is ''lower'' than the saturation-based speed, or undefined because of high noise, the saturation-based speed is specified, rounded upwards to a standard value, because using the noise-based speed would lead to overexposed images. The camera may also report the SOS-based speed (explicitly as being an SOS speed), rounded to the nearest standard speed rating.<ref name="iso12232"/>

For example, a camera sensor may have the following properties: <math>S_{40:1}=107</math>, <math>S_{10:1}=1688</math>, and <math>S_{\mathrm{sat}}=49</math>. According to the standard, the camera should report its sensitivity as
:''ISO&nbsp;100 (daylight)''
:''ISO speed latitude 50–1600''
:''ISO&nbsp;100 (SOS, daylight)''.
The SOS rating could be user controlled. For a different camera with a noisier sensor, the properties might be <math>S_{40:1}=40</math>, <math>S_{10:1}=800</math>, and <math>S_{\mathrm{sat}}=200</math>. In this case, the camera should report
:''ISO&nbsp;200 (daylight)'',
as well as a user-adjustable SOS value. In all cases, the camera should indicate for the white balance setting for which the speed rating applies, such as daylight or tungsten ([[incandescent light]]).<ref name="iso12232"/>

Despite these detailed standard definitions, cameras typically do not clearly indicate whether the user "ISO" setting refers to the noise-based speed, saturation-based speed, or the specified output sensitivity, or even some made-up number for marketing purposes. Because the 1998 version of ISO&nbsp;12232 did not permit measurement of camera output that had lossy compression, it was not possible to correctly apply any of those measurements to cameras that did not produce [[sRGB]] files in an uncompressed format such as [[Tagged Image File Format|TIFF]]. Following the publication of CIPA&nbsp;DC-004 in 2006, Japanese manufacturers of digital still cameras are required to specify whether a sensitivity rating is REI or SOS.{{Citation needed|date=April 2009}}

A greater SOS setting for a given sensor comes with some loss of image quality, just like with analog film. However, this loss is visible as image noise rather than [[film grain|grain]]. APS- and 35&nbsp;mm-sized digital [[image sensors]], both CMOS and CCD based, do not produce significant noise until about ''ISO&nbsp;1600''.<ref name="D200"/>

==See also==
* [[Frame rate]]
* [[Lens speed]]
* [[Preferred number]]

==References==
<references>
<ref name="DIN_1934_DIN-4512_Introduction">{{cite book |title=DIN 4512:1934-01. Photographische Sensitometrie, Bestimmung der optischen Dichte |publisher=[[Deutscher Normenausschuß]] (DNA) |date=1934 |language=de |quote=In the introduction to the standard, [[#Warnerke|Warnerke]]'s system is described as the first practical system used to measure emulsion speeds, but as being unreliable. In regard to Scheiner's system, it states: "Auch hier erwies sich nach einiger Zeit, daß das Meßverfahren trotz der von [[Josef Maria Eder|Eder]] vorgenommenen Abänderungen den Anforderungen der Praxis nicht vollständig Rechnung zu tragen vermag, so daß jeder Hersteller […] nach seinem eigenen System die Empfindlichkeit in [[#Scheiner|Scheinergraden]] ermitteln muß, häufig in sehr primitiver Weise durch […] Vergleich mit Erzeugnissen anderer Hersteller. Die so ermittelten Gebrauchs-Scheinergrade haben mit dem ursprünglich […] ausgearbeiteten Meßverfahren nach Scheiner sachlich nichts mehr zu tun. […] Als Folge hiervon ist allmählich eine Inflation in Empfindlichkeitsgraden eingetreten, für die das Scheiner'sche Verfahren nichts mehr als den Namen hergibt."}}</ref>
<ref name="RPS_2011_Progress_Medal_1882">{{cite book |publisher=[[Royal Photographic Society]] |title=Progress medal}}, and web-page listing people, who have received this award since 1878: {{cite web |title=Progress medal |url=http://www.rps.org/annual-awards/Progress-medal |access-date=2013-04-19 |archive-url=https://web.archive.org/web/20120822030453/http://www.rps.org/annual-awards/Progress-Medal |archive-date=2012-08-22 |quote=Instituted in 1878, this medal is awarded in recognition of any invention, research, publication or other contribution which has resulted in an important advance in the scientific or technological development of photography or imaging in the widest sense. This award also carries with it an Honorary Fellowship of The Society. […] 1882 [[Leon Warnerke]] […] 1884 [[Josef Maria Eder|J. M. Eder]] […] 1898 [[Ferdinand Hurter]] and [[Vero Charles Driffield|Vero C. Driffield]] […] 1910 [[Alfred Watkins]] […] 1912 [[Henry Chapman Jones|H. Chapman Jones]] […] 1948 [[Loyd Ancile Jones|Loyd A. Jones]]<!-- sic --> […]}}</ref>
<ref name="Jones_1911_Cyclopaedia">{{cite book |editor-first=Bernhard Edward |editor-last=Jones |title=Cassell's cyclopaedia of photography |publisher=[[Cassell (publisher)|Cassell]] |location=London, UK |date=1911 |url=https://archive.org/details/cu31924030707867}} (Reprinted as {{cite book |author-first=Bernhard Edward |author-last=Jones |title=Encyclopaedia of photography – With a New Picture Portfolio |contribution=introduction |contributor-first1=Peter C. |contributor-last1=Bunnell |contributor-link1=Peter C. Bunnell |contributor-first2=Robert A. |contributor-last2=Sobieszek |publisher=Arno Press Inc. |location=New York, USA |date=1974 |isbn=0-405-04922-6 |pages=[https://archive.org/details/cassellscyclopae0000jone/page/472 472–473] |url-access=registration |url=https://archive.org/details/cassellscyclopae0000jone/page/472 }}: 'Soon after the introduction of the gelatine dry plate, it was usual to express the speed of the emulsion as "x times", which meant that it was x times the speed of a wet collodion plate. This speed was no fixed quantity, and the expression consequently meant but little. [[#Warnerke|Warnerke]] introduced a sensitometer, consisting of a series of numbered squares with increasing quantities of opaque pigment. The plate to be tested was placed in contact with this, and an exposure made to light emanating from a tablet of luminous paint, excited by burning magnesium ribbon. After development and fixation the last number visible was taken as the speed of the plate. The chief objections to this method were that practically no two numbered tablets agreed, that the pigment possessed selective spectral absorption, and that the luminosity of the tablet varied considerably with the lapse of time between its excitation and the exposure of the plate. […] [[Henry Chapman Jones|Chapman Jones]] has introduced a modified Warnerke tablet containing a series of twenty-five graduated densities, a series of coloured squares, and a strip of neutral grey, all five being of approximately equal luminosity, and a series of four squares passing a definite portion of the spectrum; finally, there is a square of a line design, over which is superposed a half-tone negative. This "plate tester", […] is used with a standard candle as the source of light, and is useful for rough tests of both plates and printing papers.')</ref>
<ref name="Hasluck_1905_Jones">{{cite book |author-first=Paul Nooncree |author-last=Hasluck |author-link=Paul Nooncree Hasluck |date=1905 |title=The Book of Photography: Practical, Theoretical and Applied |url=https://archive.org/details/bookofphotograph00hasl |quote=THE CHAPMAN JONES PLATE TESTER. A convenient means of testing the colour rendering and other properties of a sensitive plate, or for ascertaining the effect of various colour screens, is afforded by the plate tester devised by Mr. [[Henry Chapman Jones|Chapman Jones]] in 1900. This consists of a number of graduated squares by which the sensitiveness and range of gradation of the plate examined may be determined; a series of squares of different colours and mixtures of colours of equal visual intensity, which will indicate the colour sensitiveness; and a strip of uncoloured space for comparison purposes. It is simply necessary to expose the plate being tested, in contact with the screen, to the light of a standard candle. A suitable frame and stand are supplied for the purpose; any other light may, however, be used if desired. The plate is then developed, when an examination of the negative will yield the desired information. The idea of the coloured squares is based on that of the [[William de Wiveleslie Abney|Abney]] Colour Sensitometer, where three or four squares of coloured and one of uncoloured glass are brought to an equal visual intensity by backing where necessary with squares of exposed celluloid film developed to suitable density.}}</ref>
<ref name="Sowerby">{{cite book |title=Dictionary of Photography: A Reference Book for Amateur and Professional Photographers |author-first=Arthur |author-last=Lindsay |editor-first=MacRae |editor-last=Sowerby |date=1961 |edition=19th |publisher=[[Iliffe Books Ltd.]] |location=London, UK |pages=582–589}}</ref>
<ref name="Konovalov_2007">{{cite book |title=Characteristic curve |author-first=Leonid |author-last=Konovalov |author-link=Leonid Konovalov |publisher=[[Всероссийский государственный институт кинематографии]] (ВГИК) |location=Moscow |date=2007 |page=24 |access-date=2012-11-09 |url=http://media-shoot.ru/books/Konovalov-Harakteristi4eskaja_krivaja.pdf}}</ref>
<ref name="Riat_2006">{{cite book |author-first=Martin |author-last=Riat |title=Graphische Techniken – Eine Einführung in die verschiedenen Techniken und ihre Geschichte |language=de |type=E-Book |edition=3rd German |date=Spring 2006 |url=http://www.riat-serra.org/g_tech_3.pdf}}, based on a Spanish book: {{cite book |author-first=Martin |author-last=Riat |title=Tecniques Grafiques: Una Introduccio a Les Diferents Tecniques I a La Seva Historia |language=es |edition=1st |publisher=Aubert |date=September 1983 |isbn=84-86243-00-9}}<!-- TBD: On Scheiner's system, the author writes "Scheiner ließ mit einer bestimmten Geschwindigkeit eine lichtundurchlässige Scheibe rotieren, aus der eine Öffnung herausgesägt war in der Gestalt von 20 konzentrischen Sektorringen, deren Sektorwinkel von außen nach innen eine zunehmende geometrische Folge bildeten. Die Konstante der geometrischen Folge war so gewählt, dass der letzte Winkel 100 mal größer als der erste war. Die Konstante entsprach somit der neunzehnten Wurzel aus 100, also ungefähr 1,274… Eine genormte Lichtquelle beleuchtete die Scheibe aus einer ganz bestimmten Distanz. Hinter der Scheibe war eine Kassette angebracht […], die das zu prüfende Material aufnahm. Diese Kassette wurde genau eine Minute lang geöffnet, während der die Scheibe rotierte. Die Nummer des ersten Ringes, von innen nach außen gezählt, hinter dem das Material nach der Entwicklung eine minimale, genau festgelegte Schwärzung aufwies, bestimmte den Empfindlichkeitsgrad in Scheinergraden des zu prüfenden Materials. Der letzte Grad dieser Skala (die später auf höhere Empfindlichkeitsgrade erweitert werden sollte), der Grad 20, entsprach einer 100 mal höheren Empfindlichkeit als der erste. Ein Unterschied von 3º Scheiner entspricht annähernd einer doppelten Empfindlichkeit, da die dritte Potenz der neunzehnten Wurzel von 100, nämlich 2,069…, eine Annäherung an die Zahl 2 bildet." On the DIN system he writes: "Die Empfindlichkeitsmessung im DIN-System beruht auf der Belichtung der Materialien durch einen Stufengraukeil mit einer Konstante, die so beschaffen ist, dass ihr Dreifaches dem Zehnerlogarithmus von 2 gleich ist. Diese Konstante ist also eine gute Annäherung an 0,1. Die erste Zone unter dem Graukeil, dessen Dichte sich nach dem Entwickeln des Materials um mindestens 0,1 vom unbelichteten Material unterscheidet, bestimmt dessen Empfindlichkeit. Die Belichtung erfolgt mit einem genormten Licht, das sich durch eine gewisse Annäherung an die spektrale Zusammensetzung des Tageslichts auszeichnet, was beim Scheiner-System nicht der Fall war. Die Belichtungszeit wird durch einen Schwerkraftverschluss bestimmt". --></ref>
<ref name="Sheppard_1932_Dresden-Congress">{{cite journal |author-first=Samuel Edward |author-last=Sheppard |title=Resumé of the Proceedings of the Dresden International Photographic Congress |editor-first=Sylvan |editor-last=Harris |journal=[[Journal of the Society of Motion Picture Engineers]] |publisher=[[Society of Motion Picture Engineers]] (SMPE) |volume=XVIII |number=2 |date=February 1932 |pages=232–242 |url=https://archive.org/details/journalofsociety18socirich |quote=[…] The 8th International Congress of Photography was held at Dresden, Germany, from [3 to 8] August […] 1931, inclusive. […] In regard to sensitometric standardization, several important developments occurred. First, the other national committees on sensitometric standardization accepted the light source and filter proposed by the American Committee at Paris, 1925, and accepted by the British in 1928. In the meantime, no definite agreement had been reached, nor indeed had very definite proposals been made on the subjects of sensitometers or exposure meters, development, density measurement, and methods of expressing sensitometric results, although much discussion and controversy on this subject had taken place. At the present Congress, a body of recommendations for sensitometric standards was put forward by the Deutschen<!-- sic --> Normenausschusses<!-- sic --> [für]<!-- sic --> Phototechnik, which endeavored to cover the latter questions and bring the subject of sensitometric standardization into the industrial field. It was stated by the German committee that this action had been forced on them by difficulties arising from indiscriminate and uncontrolled placing of speed numbers on photographic sensitive goods, a situation which was summarized at the Congress by the term "[[#Scheiner|Scheiner]]-inflation". The gist of these recommendations was as follows: (a) Acceptance of the light source and daylight filter as proposed by the American commission. (b) As exposure meter, a density step-wedge combined with a drop shutter accurate to 1/20<!-- difficult to decipher --> second. (c) Brush development in a tray with a prescribed solution of metol-hydroquinone according to a so-called "optimal" development. (d) Expression of the sensitivity by that illumination at which a density of 0.1 in excess of fog is reached. (e) Density measurement shall be carried out in diffused light according to details to be discussed later. These proposals aroused a very lively discussion. The American and the British delegations criticized the proposals both as a whole and in detail. As a whole they considered that the time was not ripe for application of sensitometric standards to industrial usage. In matters of detail they criticized the proposed employment of a step-wedge, and the particular sensitivity number proposed. The latter approaches very roughly the idea of an exposure for minimum gradient, but even such a number is not adequate for certain photographic uses of certain materials. The upshot of the discussion was that the German proposals in somewhat modified form are to be submitted simply as proposals of the German committee for sensitometric standardization to the various national committees for definite expression of opinion within six months of the expiration of the Congress. Further, in case of general approval of these recommendations by the other national committees, that a small International Committee on Sensitometric Standardization shall, within a further period of six months, work out a body of sensitometric practices for commercial usage.}}</ref>
<ref name="Biltz_1933_DIN-Grade">{{cite journal |author-first=Martin |author-last=Biltz |title=Über DIN-Grade, das neue deutsche Maß der photographischen Empfindlichkeit |language=de |journal=[[Naturwissenschaften (journal)|Naturwissenschaften]] |issn=0028-1042 |volume=21 |issue=41 |date=October 1933 |pages=734–736 |publisher=[[Springer-Verlag|Springer]] |doi=10.1007/BF01504271 |bibcode=1933NW.....21..734B |s2cid=31974234 |quote=[…] Im folgenden soll an Hand der seither gebräuchlichen sensitometrischen Systeme nach [[Julius Scheiner|Scheiner]] […], nach [[Hurter and Driffield|Hurter und Driffield]] […] und nach [[Josef Maria Eder|Eder]] und {{Interlanguage link multi|Walter Hecht{{!}}Hecht|de|3=Walter Hecht}} […] kurz gezeigt werden, wie man bisher verfahren ist. Im Anschlusse daran wird das neue vom Deutschen Normenausschusse für Phototechnik auf Empfehlung des Ausschusses für Sensitometrie der Deutschen Gesellschaft für photographische Forschung vorgeschlagene System […] betrachtet werden. […]}}</ref>
<ref name="Heisenberg_1930_Tagung">{{cite journal |author-first=Erwin |author-last=Heisenberg |author-link=:de:Erwin Heisenberg<!-- :wikidata:Q4111888 --> |title=Mitteilungen aus verschiedenen Gebieten – Bericht über die Gründung und erste Tagung der Deutschen Gesellschaft für photographische Forschung (23. bis 25. Mai 1930) |language=de |journal=[[Naturwissenschaften (journal)|Naturwissenschaften]] |issn=0028-1042 |volume=18 |issue=52 |date=December 1930 |pages=1130–1131 |publisher=[[Springer-Verlag|Springer]] |doi=10.1007/BF01492990 |s2cid=42242680 |quote=[…] Weitere 3 Vorträge von Prof. Dr. R. {{interlanguage link|Robert Thomas Dietrich Luther{{!}}Luther|de|Robert Thomas Dietrich Luther}}, Dresden, Prof. Dr. <!-- TBD: most probably meant was Dr. Hans Lehmann, who also designed a red light spectrograph and was involved in intensity scala sensitometry around 1900, but Berlin does not seem to match..., f.e. https://archive.org/stream/bub_gb_a83NAAAAMAAJ/bub_gb_a83NAAAAMAAJ_djvu.txt -->Lehmann, Berlin, Prof. Dr. [[Marcello Stefano Pirani|Pirani]], Berlin, behandelten die Normung der sensitometrischen Methoden. Zu normen sind: die Lichtquelle, die Art der Belichtung (zeitliche oder Intensitätsabstufung), die Entwicklung, die Auswertung. Auf den Internationalen Kongressen in Paris 1925 und London 1928 sind diese Fragen schon eingehend behandelt und in einzelnen Punkten genaue Vorschläge gemacht worden. Die Farbtemperatur der Lichtquelle soll 2360° betragen. Vor dieselbe soll ein Tageslichtfilter, welches vom [[Bureau of Standards]] ausgearbeitet worden ist, geschaltet werden. Herr Luther hat an der Filterflüssigkeit durch eigene Versuche gewisse Verbesserungen erzielt. Schwierigkeiten bereitet die Konstanthaltung der Farbtemperatur bei Nitralampen. Herr Pirani schlug deshalb in seinem Vortrag die Verwendung von Glimmlampen vor, deren Farbe von der Stromstärke weitgehend unabhängig ist. In der Frage: Zeit- oder Intensitätsskala befürworten die Herren Luther und Lehmann die Intensitätsskala. Herr Lehmann behandelte einige Fragen, die mit der Herstellung der Intensitätsskala zusammenhängen. Ausführlicher wurde noch die Auswertung (zahlenmäßige Angabe der Empfindlichkeit und Gradation) besprochen, die eine der wichtigsten Fragen der Sensitometrie darstellt. In der Diskussion wurde betont, daß es zunächst nicht so sehr auf eine wissenschaftlich erschöpfende Auswertung ankomme als darauf, daß die Empfindlichkeit der Materialien in möglichst einfacher, aber eindeutiger und für den Praktiker ausreichender Weise charakterisiert wird. […]}}</ref>
<ref name="Voss_2002_Luther">{{cite journal |author-first=Waltraud |author-last=Voss |title=Robert Luther – der erste Ordinarius für Wissenschaftliche Photographie in Deutschland – Zur Geschichte der Naturwissenschaften an der TU Dresden (12) |language=de |journal=Dresdner UniversitätsJournal |volume=13 |number=5 |page=7 |date=2002-03-12 |url=http://tu-dresden.de/die_tu_dresden/verwaltung/dezernat_5/sachgebiet_5_7/uj/bilder/pdf2002/UJ05-02.pdf |quote={{interlanguage link|Robert Thomas Dietrich Luther{{!}}Luther|de|Robert Thomas Dietrich Luther}} war Mitglied des Komitees zur Veranstaltung internationaler Kongresse für wissenschaftliche und angewandte Photographie; die Kongresse 1909 und 1931 in Dresden hat er wesentlich mit vorbereitet. 1930 gehörte er zu den Mitbegründern der Deutschen Gesellschaft für Photographische Forschung. Er gründete und leitete den Ausschuss für Sensitometrie der Gesellschaft, aus dessen Tätigkeit u.a. das DIN-Verfahren zur Bestimmung der Empfindlichkeit photographischer Materialien hervorging. […] |access-date=6 August 2011 |archive-url=https://web.archive.org/web/20110917033425/http://tu-dresden.de/die_tu_dresden/verwaltung/dezernat_5/sachgebiet_5_7/uj/bilder/pdf2002/UJ05-02.pdf |archive-date=17 September 2011 }}</ref>
<ref name="Buckland_2008_Kinamo">{{cite journal |author-link=Michael Buckland |author-first=Michael Keeble |author-last=Buckland |title=The Kinamo movie camera, Emanuel Goldberg and Joris Ivens |edition=Preprint |journal=Film History |volume=20 |number=1 |date=2008 |pages=49–58 |doi=10.2979/FIL.2008.20.1.49 |s2cid=194951687 |url=http://people.ischool.berkeley.edu/~buckland/kinamo08.pdf |quote=[[Joris Ivens|Ivens]] returned to [[Dresden]] in August 1931 to attend the VIII [[International Congress of Photography]], organised by [[Emanuel Goldberg|Goldberg]]; {{interlanguage link|John Eggert|de}}, head of research at the [[Agfa]] plant in [[ORWO|Wolfen]], near [[Leipzig]]; and {{interlanguage link|Robert Thomas Dietrich Luther{{!}}Robert Luther|de|Robert Thomas Dietrich Luther}}, the founding Director of the Institute for Scientific Photography at the Technical University in Dresden and Goldberg's dissertation advisor. The proceedings were heavily technical and dominated by discussion of the measurement of film speeds. The Congress was noteworthy because a film speed standard proposed by Goldberg and Luther was approved and, in Germany, became [[#DIN|DIN 4512]], […]}}</ref>
<ref name="Eggert_1932_Kongress-Bericht">{{cite web |url=https://books.google.com/books?id=N5kAAAAAMAAJ | editor-first1=John Emil Max |editor-last1=Eggert |editor-link1=:de:John Eggert |editor-first2=Arpad |editor-last2=von Biehler |title=Bericht über den VIII. Internationalen Kongreß für wissenschaftliche und angewandte Photographie Dresden 1931 |language=de |publisher=[[:de:Johann Ambrosius Barth Verlag|Johann Ambrosius Barth Verlag]] |location=Leipzig |date=1932}}</ref>
<ref name="Benser_1957_Farbig">{{cite book |title=Wir photographieren farbig |language=de |author-first=Walther |author-last=Benser |author-link=Walther Benser |publisher=Europäischer Buchklub |date=1957 |page=10}}</ref>
<ref name="ISO_6">{{cite book |title=ISO 6:1993: Photography – Black-and-white pictorial still camera negative film/process systems – Determination of ISO speed |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=3580}}</ref>
<ref name="http">{{cite book |title=ISO 2240:2003: Photography – Colour reversal camera films – Determination of ISO speed |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=34533}}</ref>
<ref name="iso.org">{{cite book |title=ISO 5800:1987: Photography – Colour negative films for still photography – Determination of ISO speed |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=11948}}</ref>
<ref name="Mulhern_1990_Letter">{{cite book |author-first=Charles J. |author-last=Mulhern |title=Letter to John D. de Vries |date=1990-06-15 |url=http://www.johndesq.com/westonmaster/edward.htm |type=[[Copyscript]] on John D. de Vries' web-site |archive-url=https://archive.today/20130103115812/http://www.johndesq.com/westonmaster/edward.htm |archive-date=2013-01-03 |quote= }}In 1931, Edward Faraday Weston applied for a U.S<!-- no dot here in web cite --> patent on the first Weston Exposure meter, which was granted patent No. [https://www.google.com/patents/about?id=ncNPAAAAEBAJ 2016469]{{dead link|date=June 2024|bot=medic}}{{cbignore|bot=medic}} on [8] October 1935<!-- written as "Oktober 8, 1935" in web cite -->, also an improved version was applied for and granted U.S<!-- no dot here in web cite --> patent No. [http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=2042665 2042665] {{Webarchive|url=https://web.archive.org/web/20170226053630/http://patft.uspto.gov/netacgi/nph-Parser?patentnumber=2042665 |date=2017-02-26  }} on [7th} July 1936. From 1932 to around 1967, over 36 varieties of Weston Photographic Exposure Meters were produced in large quantities and sold throughout the world, mostly by Photographic dealers or agents, which also included the Weston film speed ratings, as there were no ASA or DIN data available at that time.</ref>
<ref name="Goodwin_1938_emulsion_ratings">{{cite journal |author-first=William Nelson Jr. |author-last=Goodwin |title=Weston emulsion speed ratings: What they are and how they are determined |journal=[[American Photographer]] |date=August 1938}} (4 pages)</ref>
<ref name="Roseborough_1996_PHSC-PC-22-3">{{cite journal |author-first=Everett |author-last=Roseborough |title=The Contributions of Edward W.<!-- TBD: Certainly in the title, but can someone confirm his second surname W.? This is the only source I could find mentioning it. --> Weston and his company |journal=Photographic Canadiana |volume=22 |issue=3 |date=1996 |url=http://www.phsc.ca/excerpt22.html}}</ref>
<ref name="Tipper_Weston">{{cite web |author-first=Martin |author-last=Tipper |url=http://www.westonmeter.org.uk/thecompany.htm |title=Weston — The company and the man |work=www.westonmeter.org.uk, a web-page on Weston exposure meters |quote=[…] the Weston method of measuring film speeds. While it had some shortcomings it had the advantage of being based on a method which gave practical speeds for actual use and it was independent of any film manufacturer. Previous speed systems such as the H&D and early Scheiner speeds were both threshold speeds and capable of considerable manipulation by manufacturers. Weston's method measured the speed well up on the curve making it more nearly what one would get in actual practice. (This means that he was a bit less optimistic about film sensitivity than the manufacturers of the day who were notorious for pretending their films were more sensitive than they really were.) A certain Mr. W.<!-- no separating space here in web cite --> N. Goodwin of Weston is usually credited with this system.}}</ref>
<ref name="Hefley_1951_micrographs">{{cite journal |author-first=Harold M. |author-last=Hefley |title=A method of calculating exposures for photomicrographs |journal=[[Arkansas Academy of Science Journal]] |issue=4 |date=1951 |location=Fayetteville, USA |publisher=[[University of Arkansas]] |url=http://libinfo.uark.edu/aas/issues/1951v4/v4a28.pdf}} (NB. Research paper on an exposure system for micro-photography based on a variation of Weston film speed ratings.)</ref>
<ref name="Weston_1946_Ratings">{{cite book |publisher=Weston |title=Weston film ratings — Weston system of emulsion ratings |location=Newark, USA |date=1946 |type=Booklet, 16 pages |quote=You cannot necessarily depend on Weston speed values from any other source unless they are marked "OFFICIAL WESTON SPEEDS BY AGREEMENT WITH THE WESTON ELECTRICAL INSTRUMENT CORPORATION"}}</ref>
<ref name="Sangamo_1959_Weston-Ratings">{{cite book |publisher=Sangamo Weston |title=Weston ratings |location=Enfield, UK |date=1956 |type=Booklet, 20 pages |quote=WESTON RATINGS—Correct exposure depends on two variables: (1) the available light and (2) its effect on the film in use. WESTON have always considered these two to be of equal importance and therefore introduced their own system of film ratings. Subsequently this system was found to be so successful that it was widely accepted in photographic circles and formed the basis for internationally agreed standards.}}</ref>
<ref name="General-Electric_194x_GES-2810_Manual_GW-68">{{cite book |publisher=[[General Electric]] |title=GW-68. Manual |id=GES-2810 |location=USA}} (The manual states that ASA was working on standardized values, but none had been established at this time.)</ref>
<ref name="General-Electric_1947_GED-744_Film-Values">{{cite book |publisher=[[General Electric]] |title=General Electric Film Values |type=Leaflet, 12 pages |id=General Electric publication code GED-744 |location=USA |date=1947 |quote=This General Electric Film Value Booklet contains the […] exposure-index numbers for […] photographic films in accordance with the new system for rating photographic films that has been devised by the American Standards Association. This system has been under development for several years and is the result of co-operative effort on the part of all the film manufacturers, meter manufacturers, the Optical Society of America, and the Bureau of Standards. It was used by all of the military services during the war. The new ASA exposure-index numbers provide the photographer with the most accurate film-rating information that has yet been devised. The G-E exposure meter uses the ASA exposure-index numbers, not only in the interest of standardization, but also because this system represents a real advancement in the field of measurement. The exposure-index number have been so arranged that all earlier model G-E meters can be used with this series of numbers. For some films the values are exactly the same; and where differences exist, the new ASA exposure-index value will cause but a slight increase in exposure. However […] a comparison of the new ASA exposure-index numbers and the G-E film values is shown […] A complete comparison of all systems of emulsion speed values can be found in the G-E Photo Data Book. […] All G-E meters manufactured after January, 1946, utilize the ASA exposure indexes. Although the new ASA values can be used with all previous model G-E meters, interchangeable calculator-hoods with ASA exposure indexes are available for Types DW-48, DW-49, and DW-58 meters.}}</ref>
<ref name="General-Electric_1942_GET-I717">{{cite book |publisher=[[General Electric]] |title=General Electric Photo Data Book |id=GET-I717}}</ref>
<ref name="General-Electric_1946_Ad_Hood">{{cite web |author=General Electric |author-link=General Electric |title=Attention exposure meter owners |type=Advertisement |date=1946 |url=http://www.jollinger.com/photo/meters/images/ads/GE/ge_dw58hood%281946%29.jpg |quote=Attention! Exposure meter owners! Modernizing Hood $3.50 […] Modernize your G-E meter (Type DW-48 or early DW-58) with a new G-E Hood. Makes it easy to use the new film-exposure ratings developed by the American Standards Association … now the only basis for data published by leading film makers. See your photo dealer and snap on a new G-E hood! General Electric Company, Schenectady 5, N.Y.}}</ref>
<ref name="Gorokhovskiy_1970_Metrologiya">{{cite journal |author-first=Yu. N. |author-last=Gorokhovskiy |script-title=ru:Fotograficheskaya metrologiya |journal=Uspekhi Nauchnoy Fotografii (Advances in Scientific Photography) |language=ru |volume=15 |date=1970 |pages=183–195}} (English translation: {{cite book |title=Photographic Metrology |type=NASA Technical Translation II F-13,921, National Aeronautics and Space Administration, Washington, D.C. 20546 |date=November 1972 |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19730001888_1973001888.pdf}})</ref>
<ref name="GOST_2817-50">{{cite book |title=GOST 2817-50 Transparent sublayer photographic materials. Method of general sensitometric test |url=http://www.gostrf.com/standart/Pages_gost/46030.htm |access-date=7 August 2011 |archive-url=https://web.archive.org/web/20111011230119/http://gostrf.com/standart/Pages_gost/46030.htm |archive-date=11 October 2011 }} (GOST 2817-45 was replaced by GOST 2817-50, which in turn was replaced by GOST 10691.6–88, which defines black-and-white films, whereas GOST 10691.5–88 defines black-and-white films for aerial photography.)</ref>
<ref name="Stroebel_1993_Encyclopedia">{{cite book |title=The Focal Encyclopedia of Photography |author-first1=Leslie D. |author-last1=Stroebel |author-first2=Richard D. |author-last2=Zakia |publisher=[[Focal Press]] |date=1993 |edition=3rd |isbn=978-0-240-51417-8 |page=[https://archive.org/details/focalencyclopedi00lesl/page/304 304] |url=https://archive.org/details/focalencyclopedi00lesl|url-access=registration }}</ref>
<ref name="ZENIT_2011_GOST">{{cite web |url=https://translate.google.com/translate?hl=en&sl=ru&u=http://www.zenitcamera.com/qa/qa-filmspeeds.html |language=ru |author-first=Красногорский [Krasnogorskiy] |author-last=завод [Zavod] |author-link=Krasnogorskiy Zavod |title=Questions and answers: Film speeds |access-date=2011-08-06}}</ref>
<ref name="GOST_10691-84">{{cite book |title=GOST 10691.0–84 Black-and-white photographic materials with transparent sublaver. Method of general sensitometric test |url=http://www.gostrf.com/standart/Pages_gost/29411.htm |access-date=9 August 2011 |archive-url=https://web.archive.org/web/20120112165206/http://www.gostrf.com/standart/Pages_gost/29411.htm |archive-date=12 January 2012 }}</ref>
<ref name="GOST_10691.6-88">{{cite book |title=GOST 10691.6–88 Black-and-white phototechnical films, films for scientific researches and industry. Method for determination of speed numbers |url=http://www.gostrf.com/standart/Pages_gost/11439.htm |access-date=9 August 2011 |archive-url=https://web.archive.org/web/20120112165341/http://www.gostrf.com/standart/Pages_gost/11439.htm |archive-date=12 January 2012 }}</ref>
<ref name="GOST_10691.5-88">{{cite book |title=GOST 10691.5–88 Black-and-white aerophotographic films. Method for determination of speed numbers |url=http://www.gostrf.com/standart/Pages_gost/11651.htm |access-date=9 August 2011 |archive-url=https://web.archive.org/web/20120112165032/http://www.gostrf.com/standart/Pages_gost/11651.htm |archive-date=12 January 2012 }}</ref>
<ref name="manual">{{cite book |title=The manual of photography |author-first1=Ralph E. |author-last1=Jacobson |author-first2=Sidney F. |author-last2=Ray |author-first3=Geoffrey G. |author-last3=Attridge |author-first4=Norman R. |author-last4=Axford |publisher=[[Focal Press]] |edition=9th |date=2000 |isbn=978-0-240-51574-8 |pages=305–307 |url=https://books.google.com/books?id=MblHnLN2N2kC&pg=PA306}}</ref>
<ref name="Graves_1996_Zone-system">{{cite book |title=The zone system for 35mm photographers |author-first=Carson |author-last=Graves |publisher=[[Focal Press]] |date=1996 |isbn=978-0-240-80203-9 |page=124 |url=https://books.google.com/books?id=nHgZHXvqy5sC&pg=PA124}}</ref>
<ref name="ISO-2721_1982">{{cite web |url=http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=7692 |title=ISO 2721:1982. Photography — Cameras — Automatic controls of exposure |type=paid download |location=Geneva |publisher=[[International Organization for Standardization]] (ISO) |archive-url=https://web.archive.org/web/20080807163442/http://www.iso.org/ |archive-date=2008-08-07}}</ref>
<ref name="Leica_2002_ISO_R9">{{cite web |publisher=[[Leica Camera AG]] |date=2002 |url=http://en.leica-camera.com/assets/file/download.php?filename=file_2048.pdf |title=Leica R9 Bedienungsanleitung / Instructions |language=de, en |id=Leica publication 930 53 VII/03/GX/L |location=Solms, Germany |page=197 |quote=Film speed range: Manual setting from ISO 6/9° to ISO 12500/42° (with additional exposure compensation of up to ±3 EV, overall films from ISO 0.8/0° to ISO 100000/51° can be exposed), DX scanning from ISO 25/15° to ISO 5000/38°. |access-date=2011-07-30 |archive-date=2013-04-15  |archive-url=https://web.archive.org/web/20130415104604/http://en.leica-camera.com/assets/file/download.php?filename=file_2048.pdf |url-status=dead }}</ref>
<ref name="Leica_1996_ISO_R8">{{cite book |publisher=[[Leica Camera AG]] |date=1996 |title=Leica Instructions – Leica R8 |location=Solms, Germany |pages=16, 65<!-- |access-date=2011-07-30 --> |quote=The DX-setting for automatic speed scanning appears after the position "12800" […] Film speed range: Manual setting from ISO 6/9° to ISO 12,800/42° (With additional override of −3 EV to +3 EV, films from 0 DIN to 51 DIN can be exposed as well.) DX scanning from ISO 25/15° to ISO 5000/38°.}}</ref>
<ref name="ASA_1961_PH2-12_12500">{{cite book |title=ASA PH2.12-1961 |chapter=Table&nbsp;2 |page=9}} (NB. Showed (but did not specify) a speed of 12500 as the next full step greater than 6400.)</ref>
<ref name="Canon_1966_Booster">{{cite web |publisher=[[Canon (company)|Canon]] |url=http://www.mir.com.my/rb/photography/companies/canon/fdresources/ftql/index.htm |title=Additional Information on: Canon FT QL Camera |quote=Acceptable film speed has been increased to a range of between ASA 25 and an incredible ASA 12,800 by the use of the CANON BOOSTER. The light-measuring range of the newly developed CANON FT QL has been extended from a low of EV −3.5, f/1.2 15 seconds to EV 18 with ASA 100 film. This is the first time a TTL camera has been capable of such astonishing performance.}}</ref>
<ref name="Canon_1978_Manual_A-1">{{cite book |publisher=[[Canon (company)|Canon]] |date=1978 |title=Canon A-1 Instructions |pages=28, 29, 46, 70, 98}}</ref>
<ref name="Nikon_2009_ISO_D3s">{{cite web |work=Nikon USA Web page |url=http://www.nikonusa.com/Find-Your-Nikon/Product/Digital-SLR/25466/D3S.html |title=Nikon D3s |archive-url=https://web.archive.org/web/20120406062443/http://www.nikonusa.com/Nikon-Products/Product-Archive/Digital-SLR-Cameras/25466/D3S.html |archive-date=2012-04-06 |access-date=2010-01-11}}</ref>
<ref name="Canon_2009_ISO_EOS-1D_Mk-IV">{{cite web |work=Canon USA Web page |url=http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=139&modelid=19584#ModelTechSpecsAct |title=Canon EOS-1D Mark IV |access-date=2010-01-11 |archive-date=2017-01-06  |archive-url=https://web.archive.org/web/20170106142346/https://www.usa.canon.com/consumer/controller?act=ModelInfoAct#ModelTechSpecsAct |url-status=dead }}</ref>
<ref name="Canon_2011_ISO_EOS-1D_X">{{cite web |work=Canon USA Web page |url=http://www.usa.canon.com/cusa/consumer/products/cameras/slr_cameras/eos_1d_x |title=Canon EOS-1D X |access-date=2011-10-01 |archive-date=2014-05-08  |archive-url=https://web.archive.org/web/20140508191337/http://www.usa.canon.com/cusa/consumer/products/cameras/slr_cameras/eos_1d_x |url-status=dead }}</ref>
<ref name="Nikon_2012_ISO_D4">{{cite web |publisher=[[Nikon]] |url=http://www.nikon.com/news/2012/0106_flagship_01.htm |title=Nikon D4 |access-date=2012-01-06}}</ref>
<ref name="Ricoh_2014_ISO_Pentax_645Z">{{cite web |url=http://www.dpreview.com/news/2014/04/15/ricoh-announces-medium-format-pentax-645z |title=Ricoh Pentax 645Z specifications}}</ref>
<ref name="Nikon_2014_ISO_D4s">{{cite web |url=http://imaging.nikon.com/lineup/dslr/d4s/spec.htm |title=Nikon D4s specifications |access-date=2014-02-25  |archive-date=2014-03-02  |archive-url=https://web.archive.org/web/20140302023757/http://imaging.nikon.com/lineup/dslr/d4s/spec.htm |url-status=dead }}</ref>
<ref name="Sony_2014_ISO_Alpha_ILCE-7S">{{cite web |url=http://www.dpreview.com/news/2014/04/06/sony-announces-alpha-7-series-full-frame-mirrorless-with-4-2-2-4k-video-output?ref=title_0_1 |title=Sony α ILCE-7S specifications}}</ref>
<ref name="Canon_2015_ISO_ME20F-SH">{{cite web |title=Unsichtbares wird sichtbar! Canon präsentiert die ME20F-SH für Full-HD Farbvideos bei extrem wenig Licht |publisher=[[Canon Deutschland]] |language=de |date=2015-07-30 |work=Press release |url=http://www.canon.de/about_us/press_centre/press_releases/consumer_news/camcorders/me20f_sh_colour_video.aspx |access-date=2015-07-30 |archive-date=2015-08-02  |archive-url=https://web.archive.org/web/20150802060136/http://www.canon.de/about_us/press_centre/press_releases/consumer_news/camcorders/me20f_sh_colour_video.aspx |url-status=dead }}</ref>
<ref name="Sony_2009_ISO_Alpha_DSLR-A500-A550">{{cite web |work=Sony Europe Web page |url=http://presscentre.sony.eu/Content/Detail.aspx?NewsAreaId=2&ReleaseID=4744 |title=DSLR-A500/DSLR-A550 |date=2009-08-27 |quote=Dramatically reduced picture noise now allows super-sensitive shooting at up to ISO 12800, allowing attractive results when shooting handheld in challenging situations like candlelit interiors. |access-date=2011-07-30}}</ref>
<ref name="Sony_2010_ISO_Alpha_DSLR-A560-A580">{{cite web |work=Sony Europe Web page |url=http://presscentre.sony.eu/content/detail.aspx?ReleaseID=6109&NewsAreaId=2 |title=DSLR-A560/DSLR-A580 |archive-url=https://web.archive.org/web/20100830033236/http://presscentre.sony.eu/content/detail.aspx?ReleaseID=6109&NewsAreaId=2 |archive-date=2010-08-30 |date=2010-08-27 |quote=Multi-frame Noise Reduction 'stacks' a high-speed burst of six frames, creating a single low-noise exposure that boosts effective sensitivity as high as ISO 25600. |access-date=2011-07-30}}</ref>
<ref name="Pentax_2010_ISO_K-5">{{cite web |work=Pentax USA Web page |url=http://www.pentaximaging.com/slr/K-5/ |title=Pentax K-5 |date=2010 |quote=ISO Sensitivity: ISO 100-12800 (1, 1/2, 1/3 steps), expandable to ISO 80–51200 |access-date=2011-07-29 |archive-url=https://web.archive.org/web/20101206023448/http://www.pentaximaging.com/slr/K-5/ |archive-date=6 December 2010 }}</ref>
<ref name="Fujifilm_2011_ISO_X100">{{cite web |work=Fujifilm Canada Web page |url=http://www.fujifilm.ca/products/digital_cameras/x/finepix_x100/specifications/ |title=Fuji FinePix X100 |date=February 2011 |quote=Extended output sensitivity equivalent ISO 100 or 12800 |access-date=2011-07-30 |archive-date=2011-08-27  |archive-url=https://web.archive.org/web/20110827132520/http://www.fujifilm.ca/products/digital_cameras/x/finepix_x100/specifications/ |url-status=dead }}</ref>
<ref name="Lambrecht_2003_Way-Beyond">{{cite book |title=Way Beyond Monochrome |author-first1=Ralph W. |author-last1=Lambrecht |author-first2=Chris |author-last2=Woodhouse |publisher=Newpro UK Ltd |date=2003 |isbn=978-0-86343-354-2 |page=113 |url=https://books.google.com/books?id=Q7M0zOHcUxsC&pg=PA113}}</ref>
<ref name="Kodak_2000_Print-Grain-Index">{{cite web |url=http://www.kodak.com/global/en/professional/support/techPubs/e58/e58.jhtml |title=Kodak Tech Pub E-58: Print Grain Index |date=July 2000 |publisher=[[Eastman Kodak]], Professional Division}}</ref>
<ref name="Ilford_2006_Delta-3200">{{cite web |url=http://www.ilfordphoto.com/Webfiles/200613019405339.pdf |title=Fact Sheet, Delta 3200 Professional |location=Knutsford, U.K. |publisher=[[Ilford Photo]] |access-date=2008-05-18  |archive-date=2008-05-28  |archive-url=https://web.archive.org/web/20080528143940/http://www.ilfordphoto.com/Webfiles/200613019405339.pdf |url-status=dead }}</ref>
<ref name="iso12232">{{cite web |url=http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?CSNUMBER=37777 |title=ISO 12232:2006. Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index |location=Geneva |publisher=[[International Organization for Standardization]] (ISO) |archive-url=https://web.archive.org/web/20080807163442/http://www.iso.org/ |archive-date=2008-08-07}}</ref>
<ref name="CIPA_DC-004">{{cite web |url=http://www.cipa.jp/english/hyoujunka/kikaku/pdf/DC-004_EN.pdf |title=CIPA DC-004. Sensitivity of digital cameras. |location=Tokyo |publisher=[[Camera & Imaging Products Association]] (CIPA) |access-date=15 June 2008 |archive-url=https://web.archive.org/web/20120417120837/http://www.cipa.jp/english/hyoujunka/kikaku/pdf/DC-004_EN.pdf |archive-date=17 April 2012 }}</ref>
<ref name="Kodak_Sensor_ISO">{{cite web |url=http://www.kodak.com/global/plugins/acrobat/en/business/ISS/supportdocs/ISOMeasurements.pdf |title=Kodak Image Sensors – ISO Measurement |location=Rochester, NY, USA |publisher=[[Eastman Kodak]]}}</ref>
<ref name="CIPA_DC-008">{{cite web |url=http://www.cipa.jp/std/documents/e/DC-008-2010_E.pdf |title=Exchangeable image file format for digital still cameras: Exif Version 2.3 |publisher=CIPA |access-date=2014-12-05}}</ref>
<ref name="Kerr_2007_Digital-Camera-Sensitivity">{{cite web |url=http://dougkerr.net/Pumpkin/articles/SOS_REI.pdf |title=New Measures of the Sensitivity of a Digital Camera |author-first=Douglas A. |author-last=Kerr |date=2007-08-30}}</ref>
<ref name="ISO-12232_1998">{{cite book |title=ISO 12232:1998. Photography — Electronic still-picture cameras — Determination of ISO speed |page=12}}</ref>
<ref name="Singapore_1952">{{cite book |script-title=zh:戴淮清 《摄影入门》 |date=1952 |language=zh-hans}}</ref>
<ref name="D200">{{cite web |title=D200 Users manual |url=http://cdn-10.nikon-cdn.com/pdf/manuals/dslr/D200_en.pdf |publisher=[[Nikon]] |access-date=2015-09-20}}</ref>
<ref name="TMZ">{{cite web |title=KODAK PROFESSIONAL T-MAX Films |url=http://wwwuk.kodak.com/global/en/professional/support/techPubs/f4016/f4016.pdf |website=wwwuk.kodak.com |publisher=[[Kodak]] |access-date=2018-10-07}}</ref>
<ref name="new TMZ">{{cite web |title=KODAK PROFESSIONAL T-MAX P3200 Black & White Negative Film |url=http://imaging.kodakalaris.com/sites/prod/files/files/products/F4001.pdf |website=imaging.kodakalaris.com |publisher=[[Kodak Alaris]] |access-date=2018-10-07}}</ref>
<ref name="Ilford_2010_Delta3200">{{cite web |title=Delta 3200 Professional – technical information |date=May 2010 |website=ilfordphoto.com |publisher=[[Harman Technology]] |url=https://www.ilfordphoto.com/amfile/file/download/file_id/1872/product_id/682/ |access-date=2018-05-03 }}</ref>
<ref name="ISO 12232_2019">{{cite web |title=ISO 12232:2019 — Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index |date=February 2019 |publisher=[[International Organization for Standardization]] (ISO) |url=https://www.iso.org/obp/ui/#iso:std:iso:12232:ed-3:v1:en}}</ref>
<ref name="Vision_2016_Boosting">{{cite web |title=Boosting Sensitivity |author=<!-- no by-line --> |publisher=[[Vision Research (company)|Vision Research]] |work=[[Phantom (high-speed camera brand)|Phantom]]/[[Ametek]] |series=Notes/Alerts |date=April 2016 |location=Wayne, NJ, USA |url=https://phantomhighspeed-service.force.com/servlet/servlet.FileDownload?file=00P1N00000dSuGDUA0&oid=00D1N000002Br2EUAS |access-date=2019-06-09 |archive-date=2019-06-09  |archive-url=https://web.archive.org/web/20190609211243/https://phantomhighspeed-service.force.com/servlet/servlet.FileDownload?file=00P1N00000dSuGDUA0&oid=00D1N000002Br2EUAS |url-status=dead }}</ref>
<ref name="Gasiorowski-Denis_2019_ISO12232">{{cite web |title=A better picture: International Standard gives photography a new exposure |author-first=Elizabeth |author-last=Gasiorowski-Denis |publisher=[[International Organization for Standardization]] (ISO) |date=2019-03-07 |url=https://www.iso.org/news/ref2375.html |access-date=2019-06-09 |url-status=live |archive-url=https://archive.today/20190609201259/https://www.iso.org/news/ref2375.html |archive-date=9 June 2019 }}</ref>
</references>

==Further reading==
{{refbegin}}
* [http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=3579 ISO 6:1974], [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=3580 ISO 6:1993] (1993-02). ''Photography — Black-and-white pictorial still camera negative film/process systems — Determination of ISO speed''. Geneva: International Organization for Standardization.
* [http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=7049 ISO 2240:1982] (1982-07), [http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=7050 ISO 2240:1994] (1994-09), [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=34533 ISO 2240:2003] (2003–10). ''Photography — Colour reversal camera films — Determination of ISO speed''. Geneva: International Organization for Standardization.
* [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=7690 ISO 2720:1974]. ''General Purpose Photographic Exposure Meters (Photoelectric Type) — Guide to Product Specification''. Geneva: International Organization for Standardization.
* [http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=11947 ISO 5800:1979], [http://www.iso.org/iso/en/CatalogueDetailPage.CatalogueDetail?csnumber=11948 ISO 5800:1987] (1987-11), [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=35540 ISO 5800:1987/Cor 1:2001] (2001-06). ''Photography — Colour negative films for still photography — Determination of ISO speed''. Geneva: International Organization for Standardization.
* [http://www.iso.org/iso/iso_catalogue/catalogue_ics/catalogue_detail_ics.htm?csnumber=21020 ISO 12232:1998] (1998-08), [http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=37777 ISO 12232:2006] (2006-04-15), ISO 12232:2006 (2006-10-01), [https://www.iso.org/obp/ui/#iso:std:iso:12232:ed-3:v1:en ISO 12232:2019] (2019-02-01). ''Photography — Digital still cameras — Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index''. Geneva: International Organization for Standardization.
* ASA Z38.2.1-1943, ASA Z38.2.1-1946, ASA Z38.2.1-1947 (1947-07-15). ''American Standard Method for Determining Photographic Speed and Speed Number''. New York: American Standards Association. Superseded by ASA PH2.5-1954.
* ASA PH2.5-1954, ASA PH2.5-1960. ''American Standard Method for Determining Speed of photographic Negative Materials (Monochrome, Continuous Tone)''. New York: United States of America Standards Institute (USASI)<!-- American Standards Association before 1966. -->. Superseded by ANSI PH2.5-1972.
* ANSI PH2.5-1972, ANSI PH2.5-1979 (1979-01-01), ANSI PH2.5-1979(R1986). ''Speed of photographic negative materials (monochrome, continuous tone, method for determining)''. New York: American National Standards Institute. Superseded by NAPM IT2.5-1986.
* NAPM IT2.5-1986, [http://webstore.ansi.org/RecordDetail.aspx?sku=ANSI%2FISO+6-1993+ANSI%2FNAPM+IT2.5-1993 ANSI/ISO 6-1993 ANSI/NAPM IT2.5-1993] (1993-01-01). ''Photography — Black-and-White Pictorial Still Camera Negative Film/Process Systems — Determination of ISO Speed (same as ANSI/ISO 6-1993)''. National Association of Photographic Manufacturers. This represents the US adoption of ISO 6.
* ASA PH2.12-1957, ASA PH2.12-1961. ''American Standard, General-Purpose Photographic Exposure Meters (photoelectric type)''. New York: American Standards Association. Superseded by ANSI PH3.49-1971.
* ANSI PH2.21-1983 (1983-09-23), ANSI PH2.21-1983(R1989). ''Photography (Sensitometry) Color reversal camera films – Determination of ISO speed''. New York: American Standards Association. Superseded by ANSI/ISO 2240-1994 ANSI/NAPM IT2.21-1994.
* ANSI/ISO 2240-1994 ANSI/NAPM IT2.21-1994. ''Photography – Colour reversal camera films – determination of ISO speed''. New York: American National Standards Institute. This represents the US adoption of ISO 2240.
* ASA PH2.27-1965 (1965-07-06), ASA PH2.27-1971, ASA PH2.27-1976, ANSI PH2.27-1979, ANSI PH2.27-1981, ANSI PH2.27-1988 (1988-08-04). ''Photography – Colour negative films for still photography – Determination of ISO speed (withdrawn)''. New York: American Standards Association. Superseded by ANSI IT2.27-1988.
* ANSI IT2.27-1988 (1994-08/09?). ''Photography Color negative films for still photography – Determination of ISO speed''. New York: American National Standards Institute. Withdrawn. This represented the US adoption of ISO 5800.
* ANSI PH3.49-1971, ANSI PH3.49-1971(R1987). ''American National Standard for general-purpose photographic exposure meters (photoelectric type)''. New York: American National Standards Institute. After several revisions, this standard was withdrawn in favor of ANSI/ISO 2720:1974.
* ANSI/ISO 2720:1974, ANSI/ISO 2720:1974(R1994) ANSI/NAPM IT3.302-1994. ''General Purpose Photographic Exposure Meters (Photoelectric Type) — Guide to Product Specification''. New York: American National Standards Institute. This represents the US adoption of ISO 2720.
* BSI BS 1380:1947, BSI BS 1380:1963. ''Speed and exposure index''<!-- ... of ... -->. British Standards Institution. Superseded by BSI BS 1380-1:1973 (1973-12), BSI BS 1380-2:1984 (1984-09), BSI BS 1380-3:1980 (1980-04) and others.
* [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000040980 BSI BS 1380-1:1973] {{Webarchive|url=https://web.archive.org/web/20111009042715/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000040980 |date=2011-10-09 }} (1973-12-31). ''Speed of sensitized photographic materials: Negative monochrome material for still and cine photography''. British Standards Institution. Replaced by BSI BS ISO 6:1993, superseded by BSI BS ISO 2240:1994.
* [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000049468 BSI BS 1380-2:1984 ISO 2240:1982] {{Webarchive|url=https://web.archive.org/web/20111009042747/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000049468 |date=2011-10-09 }} (1984-09-28). ''Speed of sensitized photographic materials. Method for determining the speed of colour reversal film for still and amateur cine photography''. British Standards Institution. Superseded by BSI BS ISO 2240:1994.
* [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000040992 BSI BS 1380-3:1980 ISO 5800:1979] {{Webarchive|url=https://web.archive.org/web/20111009042901/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000040992 |date=2011-10-09 }} (1980-04-30). ''Speed of sensitized photographic materials. Colour negative film for still photography''. British Standards Institution. Superseded by BSI BS ISO 5800:1987.
<!-- * BSI BS 1380-4.1:1979 (Sec.4.1-1979), BSI BS 1380-4.1:1979 (Sec.4.2-1979), BSI BS 1380-4.3:1979 (Sec.4.3-1979), [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000158557 BSI BS 1380-4-1986 ISO 7589:1984] (1986-08-29). ''Speed of sensitized photographic materials. Specification for daylight and incandescent tungsten illuminants for sensitometry''. British Standards Institution. Superseded by BSI BS ISO 7589:2002. -->
<!-- * [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000137916 BSI BS 1380-5-1985 ISO 7187:1983] (1985-07-31). ''Speed of sensitized photographic materials. Method for determining the speed of direct positive colour print camera materials''. British Standards Institution. Superseded by BSI BS ISO 7187:1995. -->
* [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000483185 BSI BS ISO 6:1993] {{Webarchive|url=https://web.archive.org/web/20111009044139/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000483185 |date=2011-10-09 }} (1995-03-15). ''Photography. Black-and-white pictorial still camera negative film/process systems. Determination of ISO speed''. British Standards Institution. This represents the British adoption of ISO 6:1993.
* [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000497946 BSI BS ISO 2240:1994] {{Webarchive|url=https://web.archive.org/web/20111009044151/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000000497946 |date=2011-10-09 }} (1993-03-15), [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000030046567 BSI BS ISO 2240:2003] {{Webarchive|url=https://web.archive.org/web/20111009044219/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000030046567 |date=2011-10-09 }} (2004-02-11). ''Photography. Colour reversal camera films. Determination of ISO speed''. British Standards Institution. This represents the British adoption of ISO 2240:2003.
* [http://shop.bsigroup.com/en/ProductDetail/?pid=000000000030050685 BSI BS ISO 5800:1987] {{Webarchive|url=https://web.archive.org/web/20111009044326/http://shop.bsigroup.com/en/ProductDetail/?pid=000000000030050685 |date=2011-10-09 }} (1995-03-15). ''Photography. Colour negative films for still photography. Determination of ISO speed''. British Standards Institution. This represents the British adoption of ISO 5800:1987.
* DIN 4512:1934-01, DIN 4512:1957-11 (Blatt 1), DIN 4512:1961-10 (Blatt 1). ''Photographische Sensitometrie, Bestimmung der optischen Dichte''. Berlin: Deutscher Normenausschuß<!-- using 'ß' instead of 'ss' according to old German spelling --> (DNA). Superseded by DIN 4512-1:1971-04, DIN 4512-4:1977-06, DIN 4512-5:1977-10 and others.
* [http://www.beuth.de/langanzeige/DIN-4512-1-1971-04/en/1830007.html DIN 4512-1:1971-04] {{Webarchive|url=https://web.archive.org/web/20110930044229/http://www.beuth.de/langanzeige/DIN-4512-1-1971-04/en/1830007.html |date=2011-09-30  }}, [http://www.beuth.de/langanzeige/DIN-4512-1-1993-05/en/3236175.html DIN 4512-1:1993-05] {{Webarchive|url=https://web.archive.org/web/20110930044247/http://www.beuth.de/langanzeige/DIN-4512-1-1993-05/en/3236175.html |date=2011-09-30  }}. ''Photographic sensitometry; systems of black and white negative films and their process for pictorial photography; determination of speed''. Berlin: Deutsches Institut für Normung (before 1975: Deutscher Normenausschuß<!-- using 'ß' instead of 'ss' according to old German spelling --> (DNA)). Superseded by DIN ISO 6:1996-02.
<!-- * [http://www.beuth.de/langanzeige/DIN-4512-3/en/1830099.html DIN 4512-3:1973-01]. ''Photographic sensitometry, determination of diffuse transmission density''. Berlin: Deutscher Normenausschuß (DNA). Superseded by DIN 4512-7:1993-01, DIN 4512-8:1993-01, DIN 4512-9:1993-01. -->
* [http://www.beuth.de/langanzeige/DIN-4512-4-1977-06/en/1846945.html DIN 4512-4:1977-06] {{Webarchive|url=https://web.archive.org/web/20110930044259/http://www.beuth.de/langanzeige/DIN-4512-4-1977-06/en/1846945.html |date=2011-09-30  }}, [http://www.beuth.de/langanzeige/DIN-4512-4-1985-08/en/1225729.html DIN 4512-4:1985-08] {{Webarchive|url=https://web.archive.org/web/20110930044322/http://www.beuth.de/langanzeige/DIN-4512-4-1985-08/en/1225729.html |date=2011-09-30  }}. ''Photographic sensitometry; determination of the speed of colour reversal films''. Berlin: Deutsches Institut für Normung. Superseded by DIN ISO 2240:1998-06.
* [http://www.beuth.de/langanzeige/DIN-4512-5-1977-10/en/1871467.html DIN 4512-5:1977-10] {{Webarchive|url=https://web.archive.org/web/20110930044334/http://www.beuth.de/langanzeige/DIN-4512-5-1977-10/en/1871467.html |date=2011-09-30  }}, [http://www.beuth.de/langanzeige/DIN-4512-5-1990-11/en/1622790.html DIN 4512-5:1990-11] {{Webarchive|url=https://web.archive.org/web/20110930044432/http://www.beuth.de/langanzeige/DIN-4512-5-1990-11/en/1622790.html |date=2011-09-30  }}. ''Photographic sensitometry; determination of the speed of colour negative films''. Berlin: Deutsches Institut für Normung. Superseded by DIN ISO 5800:1998-06.
<!-- * DIN 4512-6:1980-07. ''Photographic sensitometry; sensitometric illuminants and light sources''. Berlin: Deutsches Institut für Normung. Withdrawn. -->
<!-- * [http://www.beuth.de/langanzeige/DIN-4512-7/en/2002338.html DIN 4512-7:1993-01]. ''Photographic sensitometry, determination of optical density; concepts, symbols and notations''. Berlin: Deutsches Institut für Normung. -->
<!-- * [http://www.beuth.de/langanzeige/DIN-4512-8/en/2002603.html DIN 4512-8:1993-01]. ''Photographic sensitometry, determination of optical density; geometric conditions for transmission density''. Berlin: Deutsches Institut für Normung. -->
<!-- * [http://www.beuth.de/langanzeige/DIN-4512-9/en/2002672.html DIN 4512-9:1993-01]. ''Photographic sensitometry, determination of optical density; spectral conditions''. Deutsches Institut für Normung. -->
<!-- * [http://www.beuth.de/langanzeige/DIN-4512-10/en/2002753.html DIN 4512-10:1993-01]. ''Photographic sensitometry, determination of optical density; geometric conditions for reflection density''. Berlin: Deutsches Institut für Normung. -->
* [http://www.beuth.de/langanzeige/DIN-ISO-6-1996-02/en/2699500.html DIN ISO 6:1996-02] {{Webarchive|url=https://web.archive.org/web/20110930044500/http://www.beuth.de/langanzeige/DIN-ISO-6-1996-02/en/2699500.html |date=2011-09-30  }}. ''Photography – Black-and-white pictorial still camera negative film/process systems – Determination of ISO speed (ISO 6:1993)''. Berlin: Deutsches Institut für Normung. This represents the German adoption of ISO 6:1993.
* [http://www.beuth.de/langanzeige/DIN-ISO-2240-1998-06/en/3367243.html DIN ISO 2240:1998-06] {{Webarchive|url=https://web.archive.org/web/20110920101022/http://www.beuth.de/langanzeige/DIN-ISO-2240-1998-06/en/3367243.html |date=2011-09-20  }}, [http://www.beuth.de/langanzeige/DIN-ISO-2240-2005-10/en/82165961.html DIN ISO 2240:2005-10] {{Webarchive|url=https://web.archive.org/web/20110930044533/http://www.beuth.de/langanzeige/DIN-ISO-2240-2005-10/en/82165961.html |date=2011-09-30  }}. ''Photography – Colour reversal camera films – Determination of ISO speed (ISO 2240:2003)''. Berlin: Deutsches Institut für Normung. This represents the German adoption of ISO 2240:2003.
* [http://www.beuth.de/langanzeige/DIN-ISO-5800-1998-06/en/3367409.html DIN ISO 5800:1998-06] {{Webarchive|url=https://web.archive.org/web/20110920082447/http://www.beuth.de/langanzeige/DIN-ISO-5800-1998-06/en/3367409.html |date=2011-09-20  }}, [http://www.beuth.de/langanzeige/DIN-ISO-5800/en/66332092.html DIN ISO 5800:2003-11] {{Webarchive|url=https://web.archive.org/web/20110930044615/http://www.beuth.de/langanzeige/DIN-ISO-5800/en/66332092.html |date=2011-09-30  }}. ''Photography – Colour negative films for still photography – Determination of ISO speed (ISO 5800:1987 + Corr. 1:2001)''. Berlin: Deutsches Institut für Normung. This represents the German adoption of ISO 5800:2001.
* Leslie B. Stroebel, John Compton, Ira Current, Richard B. Zakia. ''Basic Photographic Materials and Processes'', second edition. Boston: Focal Press, 2000. {{ISBN|0-240-80405-8}}.
{{refend}}

==External links==
{{commons category|Film speed}}
* [http://www.cs.duke.edu/~parr/photography/faq.html#isomeaning What is the meaning of ISO for digital cameras?] Digital Photography FAQ
* [http://www.cs.tut.fi/~foi/sensornoise.html Signal-dependent noise modeling, estimation, and removal for digital imaging sensors]

* [https://archive.org/details/film-speed-comparisons-version-1.4 "Handbook of Photography" by Henney and Dudley (1939)] Spreadsheet Comparing Film Speed Systems

{{photography subject|state=uncollapsed}}
{{ISO standards}}

{{DEFAULTSORT:Film Speed}}
[[Category:Science of photography]]
[[Category:Physical quantities]]