Editing Film speed (section)

==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 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 Dr. [[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 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. -->
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.

{| class="wikitable center" 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)."
|+ Table&nbsp;1. Comparison of various film speed scales
! style="background:#FFDEAD;" | [[#APEX|APEX]] ''S<sub>v</sub>'' (1960–)<!-- ASA/ANSI APEX Sv values are written without degree symbol, however, logarithmic ASA degrees were written with degree symbol. -->
! style="background:#FFDEAD;" | [[#ISO|ISO]] (1974–)<br />arith./log.°<!-- 1974 as per ISO 6:1974 -->
! style="background:#FFDEAD;" | Camera mfrs. (2009–)<!-- speeds for SLR manufacurers, which don't fit into the official scales -->
! style="background:#FFDEAD;" | [[#ASA|ASA]] (1960–1987)<br />arith.<!-- ASA values before 1960 are not compatible with ASA values since 1960 for B&W negative film. -->
! style="background:#FFDEAD;" | [[#DIN|DIN]] (1961–2002)<br />log.<!-- DIN values before 1961 are not compatible with DIN values since 1961 for B&W negative film. DIN values are written without degree symbol (°) since 1961. -->
! style="background:#FFDEAD;" | [[#GOST|GOST]] (1951–1986)<!-- 1986 for "old GOST" GOST 2817-50, since "new" GOST 10691-84 became effective on 1987-01-01. Have there been any revisions of GOST between 1951 and 1987, which would render this comparison invalid? --><br />arith.
! style="background:#FFDEAD;" | Example of film stock<br />with this nominal speed
|- style="background:#F2F2F2"
| −2
| 0.8/0°<ref name="Leica_2002_ISO_R9"/>
| &nbsp;
| 0.8
| 0<ref name="Leica_1996_ISO_R8"/>
| &nbsp;
| align="left" | FPP&nbsp;BW 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>
|-
| &nbsp;
| 1/1°
| &nbsp;
| 1
| 1
| (1)
| align="left" | Svema Micrat-orto, Astrum Micrat-orto
|-
| &nbsp;
| 1.2/2°
| &nbsp;
| 1.2
| 2
| (1)
| &nbsp;
|- style="background:#F2F2F2"
| −1
| 1.6/3°
| &nbsp;
| 1.6
| 3
| 1.4
| &nbsp;
|-
| &nbsp;
| 2/4°
| &nbsp;
| 2
| 4
| (2)
| &nbsp;
|-
| &nbsp;
| 2.5/5°
| &nbsp;
| 2.5
| 5
| (2)
| &nbsp;
|- style="background:#F2F2F2"
| 0
| 3/6°
| &nbsp;
| 3
| 6
| 2.8
| align="left" | Svema&nbsp;MZ-3, Astrum&nbsp;MZ-3
|-
| &nbsp;
| '''4/7°'''
| &nbsp;
| '''4'''
| 7
| (4)
| &nbsp;
|-
| &nbsp;
| '''5/8°'''
| &nbsp;
| '''5'''
| 8
| (4)
| align="left" | Original three-strip [[Technicolor#Convincing Hollywood|Technicolor]]
|- style="background:#F2F2F2"
| '''1'''
| '''6/9°'''
| &nbsp;
| '''6'''
| 9
| 5.5
| align="left" | Original [[Kodachrome]]
|-
| &nbsp;
| '''8/10°'''
| &nbsp;
| '''8'''
| 10
| (8)
| align="left" | Polaroid PolaBlue
|-
| &nbsp;
| '''10/11°'''
| &nbsp;
| '''10'''
| 11
| (8)
| align="left" | [[Kodachrome|Kodachrome 8&nbsp;mm film]]
|- style="background:#F2F2F2"
| '''2'''
| '''12/12°'''
| &nbsp;
| '''12'''
| 12
| 11
| align="left" | Gevacolor 8&nbsp;mm reversal film, later Agfa Dia-Direct
|-
| &nbsp;
| '''16/13°'''
| &nbsp;
| '''16'''
| 13
| (16)
| align="left" | Agfacolor 8&nbsp;mm reversal film
|-
| &nbsp;
| '''20/14°'''
| &nbsp;
| '''20'''
| 14
| (16)
| align="left" | Adox CMS&nbsp;20
|- style="background:#F2F2F2"
| '''3'''
| '''25/15°'''
| &nbsp;
| '''25'''
| 15
| 22
| align="left" | Old Agfacolor, Kodachrome&nbsp;II and (later) [[Kodachrome|Kodachrome&nbsp;25]], Efke&nbsp;25
|-
| &nbsp;
| '''32/16°'''
| &nbsp;
| '''32'''
| 16
| (32)
| align="left" | Kodak Panatomic-X
|-
| &nbsp;
| '''40/17°'''
| &nbsp;
| '''40'''
| 17
| (32)
| align="left" | [[Kodachrome|Kodachrome&nbsp;40 (movie)]]
|- style="background:#F2F2F2"
| '''4'''
| '''50/18°'''
| &nbsp;
| '''50'''
| 18
| 45
| align="left" | Fuji RVP ([[Velvia]]), Ilford Pan F Plus, Kodak Vision2&nbsp;50D&nbsp;5201 (movie), AGFA&nbsp;CT18, Efke&nbsp;50, Polaroid type&nbsp;55
|-
| &nbsp;
| '''64/19°'''
| &nbsp;
| '''64'''
| 19
| (65)
| align="left" | [[Kodachrome|Kodachrome 64]], Ektachrome-X, Polaroid type&nbsp;64T
|-
| &nbsp;
| '''80/20°'''
| &nbsp;
| '''80'''
| 20
| (65)
| align="left" | Ilford Commercial Ortho, Polaroid type&nbsp;669
|- style="background:#F2F2F2"
| '''5'''
| '''100/21°'''
| &nbsp;
| '''100'''
| 21
| 90
| align="left" | [[Kodacolor (still photography)|Kodacolor Gold]], [[Kodak T-MAX]]&nbsp;100 (TMX), Kodak [[Ektar]], [[Fujichrome Provia 100F|Fujichrome Provia&nbsp;100F]], Efke&nbsp;100, Fomapan/Arista&nbsp;100, Kentmere&nbsp;Pan&nbsp;100
|-
| &nbsp;
| '''125/22°'''
| &nbsp;
| '''125'''
| 22
| (130)
| align="left" | [[Ilford FP4+|Ilford&nbsp;FP4+]], Kodak Plus-X Pan, Svema Color&nbsp;125
|-
| &nbsp;
| '''160/23°'''
| &nbsp;
| '''160'''
| 23
| (130)
| align="left" | [[Fujicolor Pro|Fujicolor Pro&nbsp;160C/S]], Kodak High-Speed Ektachrome, [[Kodak Portra]]&nbsp;160NC and 160VC
|- style="background:#F2F2F2"
| '''6'''
| '''200/24°'''
| &nbsp;
| '''200'''
| 24
| 180
| align="left" | Kodak Gold&nbsp;200, [[Fujifilm Superia|Fujicolor Superia]]&nbsp;200, [[Agfa Scala 200x|Agfa Scala&nbsp;200x]], Fomapan/Arista&nbsp;200, Wittner Chrome&nbsp;200D/Agfa Aviphot Chrome&nbsp;200&nbsp;PE1<!-- Available in 2014. -->
|-
| &nbsp;
| '''250/25°'''
| &nbsp;
| '''250'''
| 25
| (250)
| align="left" | Tasma Foto-250, Eastman Double-X
|-
| &nbsp;
| '''320/26°'''
| &nbsp;
| '''320'''
| 26
| (250)
| align="left" | Kodak Tri-X Pan Professional (TXP)
|- style="background:#F2F2F2"
| '''7'''
| '''400/27°'''
| &nbsp;
| '''400'''
| 27
| 350
| align="left" | Kodak T-Max&nbsp;400 (TMY), [[Kodak Tri-X 400|Kodak Tri-X&nbsp;400]], [[Kodak Portra]]&nbsp;400, [[Ilford HP5+|Ilford&nbsp;HP5+]], [[Fujifilm Superia X-tra 400|Fujifilm Superia&nbsp;X-tra&nbsp;400]], [[Fujichrome Provia 400X|Fujichrome Provia&nbsp;400X]], Fomapan/Arista&nbsp;400, Kentmere&nbsp;Pan&nbsp;400
|-
| &nbsp;
| '''500/28°'''
| &nbsp;
| '''500'''
| 28
| (500)
| align="left" | Kodak Vision3&nbsp;500T&nbsp;5219 (movie)
|-
| &nbsp;
| '''640/29°'''
| &nbsp;
| '''640'''
| 29
| (500)<!-- (560) -->
| align="left" | Polaroid&nbsp;600
|- style="background:#F2F2F2"
| '''8'''
| '''800/30°'''
| &nbsp;
| '''800'''
| 30
| 700
| align="left" | [[Fujicolor Pro|Fuji Pro&nbsp;800Z]], [[Instax|Fuji Instax]]
|-
| &nbsp;
| '''1000/31°'''
| &nbsp;
| '''1000'''
| 31
| (1000)
| align="left" | [[Ilford Delta]] 3200, [[Kodak T-MAX|Kodak&nbsp;P3200 TMAX]]<ref name="TMZ"/> 
Kodak Professional T-Max P3200<ref name="new TMZ"/> <small>(see [[#Marketing anomalies|Marketing anomalies]] below)</small>
|-
| &nbsp;
| '''1250/32°'''
| &nbsp;
| '''1250'''
| 32
| (1000)
| align="left" | Kodak Royal-X Panchromatic
|- style="background:#F2F2F2"
| '''9'''
| '''1600/33°'''
| &nbsp;
| '''1600'''
| 33
| 1400 (1440)
| align="left" | Fujicolor&nbsp;1600, [[Fujifilm_Superia|Fuji Natura&nbsp;1600 and Superia&nbsp;1600]], [[Neopan#Neopan 1600 Professional (Super Presto)|Neopan&nbsp;1600]]
|-
| &nbsp;
| '''2000/34°'''
| &nbsp;
| '''2000'''
| 34
| (2000)
| &nbsp;
|-
| &nbsp;
| '''2500/35°'''
| &nbsp;
| '''2500'''
| 35
| (2000)
| &nbsp;
<!--
|-
|  
|  
| 3000
| ~36
|  
| align="left" | Polaroid Black and White 'Peel apart' Film
-->
|- style="background:#F2F2F2"
| '''10'''
| '''3200/36°'''
| &nbsp;
| '''3200'''
| 36
| 2800 (2880)
| align="left" | Konica&nbsp;3200, Polaroid type&nbsp;667, Fujifilm&nbsp;FP-3000B, Kodak Tmax&nbsp;3200&nbsp;B&W{{Note|Tmax 3200 is not actually rated to ISO standards at 3200, but delivers useful results when exposed at 3200.}}
|-
| &nbsp;
| '''4000/37°'''
| &nbsp;
| &nbsp;
| 37
| (4000)
| &nbsp;
|-
| &nbsp;
| '''5000/38°'''
| &nbsp;
| &nbsp;
| 38
| (4000)
| &nbsp;
|- style="background:#F2F2F2"
| 11
| '''6400/39°'''
| &nbsp;
| 6400<ref name="ASA_1961_PH2-12_12500"/>
| 39
| 5600
| &nbsp;
|-
| &nbsp;
| '''8000/40°'''<ref name="Leica_2002_ISO_R9"/><ref name="Leica_1996_ISO_R8"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| '''10000/41°'''<ref name="Leica_2002_ISO_R9"/><ref name="Leica_1996_ISO_R8"/><ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 12
| 12500/42°<ref name="Leica_2002_ISO_R9"/><ref name="Vision_2016_Boosting"/>
| 12800<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"/>
| &nbsp; 12500<ref name="ASA_1961_PH2-12_12500"/>
| &nbsp;
| &nbsp;
| align="left" | ISO speeds greater than 10000 have not been defined officially before ISO&nbsp;12232:2019.<ref name="ISO 12232_2019"/>
|-
| &nbsp;
| 16000/43°<ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 20000/44°<ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" | Polaroid type&nbsp;612<ref>{{cite news |last1=Manning |first1=Jack |title=CAMERA; NEW DEVELOPMENTS IN INSTANT PHOTOGRAPHY |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:#F2F2F2"
| 13
| 25000/45°<ref name="Vision_2016_Boosting"/>
| 25600<ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 32000/46°<ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 40000/47°<ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 14
| 50000/48°<ref name="Vision_2016_Boosting"/>
| 51200<ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 64000/49°<ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 80000/50°<ref name="Vision_2016_Boosting"/>
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 15
| 100000/51°<ref name="Leica_2002_ISO_R9"/>
| 102400<ref name="Nikon_2009_ISO_D3s"/><ref name="Canon_2009_ISO_EOS-1D_Mk-IV"/>
| &nbsp;
| 51<ref name="Leica_1996_ISO_R8"/>
| &nbsp;
| align="left" | <!-- List only the first commercial cameras with this ISO equivalent per manufacturer/brand. -->[[Nikon D3s|Nikon&nbsp;D3s]] and [[Canon EOS-1D Mark IV|Canon&nbsp;EOS-1D Mark&nbsp;IV]] (2009)
|-
| &nbsp;
| 125000/52°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 160000/53°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 16
| 200000/54°
| 204800<ref name="Canon_2011_ISO_EOS-1D_X"/><ref name="Nikon_2012_ISO_D4"/><ref name="Ricoh_2014_ISO_Pentax_645Z"/>
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" | <!-- List only the first commercial cameras with this ISO equivalent per manufacturer/brand. -->[[Canon EOS-1D X|Canon&nbsp;EOS-1D&nbsp;X]] (2011<!-- Availability spring 2012. -->), [[Nikon D4|Nikon&nbsp;D4]] (2012<!-- Availability spring 2012. -->), [[Pentax 645Z|Pentax&nbsp;645Z]] (2014<!-- Availability 2014-07. -->)
|-
| &nbsp;
| 250000/55°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 320000/56°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 17
| 400000/57°
| 409600<ref name="Nikon_2014_ISO_D4s"/><ref name="Sony_2014_ISO_Alpha_ILCE-7S"/>
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" | <!-- List only the first commercial digital cameras with this ISO equivalent per manufacturer/brand. -->[[Nikon D4s|Nikon&nbsp;D4s]], [[Sony α ILCE-7S|Sony&nbsp;α ILCE-7S]] (2014), [[Canon EOS 1D X Mark II|Canon&nbsp;EOS&nbsp;1D&nbsp;X Mark&nbsp;II]] (2016)
|- 
| &nbsp;
| 500000/58°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|-
| &nbsp;
| 640000/59°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 18
| 800000/60°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- 
| &nbsp;
| 1000000/61°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" |<!-- List only the first commercial digital cameras with this ISO equivalent per manufacturer/brand. -->
|-
| &nbsp;
| 1250000/62°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 19
| 1600000/63°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- 
| &nbsp;
| 2000000/64°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" |<!-- List only the first commercial digital cameras with this ISO equivalent per manufacturer/brand. -->
|-
| &nbsp;
| 2500000/65°
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
|- style="background:#F2F2F2"
| 20
| 3200000/66°
| 3280000&nbsp;
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" | <!-- List only the first commercial digital cameras with this ISO equivalent per manufacturer/brand. -->[[Nikon D5|Nikon&nbsp;D5]] (2016)
|- 
| &nbsp;
| 4000000/67°<ref name="Canon_2015_ISO_ME20F-SH"/>
| 4560000
| &nbsp;
| &nbsp;
| &nbsp;
| align="left" |<!-- List only the first commercial digital cameras with this ISO equivalent per manufacturer/brand -->[[Canon ME20F-SH|Canon&nbsp;ME20F-SH]]<ref name="Canon_2015_ISO_ME20F-SH"/> (2015)
|-
|21
|
|104857600
|
|
|
| align="left" | [https://www.photonis.com/news/photonis-launches-inocturn-first-intensified-camera-combining-photonis-image-intensifier-tube Photonis INocturn]<ref>{{Cite web |title=iNocturn: combining the best of image intensifier tube and low light camera |url=https://www.photonis.com/innovation-researches-and-innovations/inocturn-combining-best-image-intensifier-tube-and-low-light |access-date=2022-03-25 |website=Photonis |language=en}}</ref> (2021)
|}

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}}===
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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.