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Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the 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 in the U.S. and DIN in Europe.
The term speed comes from the early days of photography. Photographic emulsions 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 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, the use of higher sensitivities generally leads to reduced image quality (via coarser film grain or higher image noise). Generally, the higher the sensitivity, the grainier the image will be. 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
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). [1]
Threshold
The threshold criterion is the point on the characteristic curve corresponding to just perceptible density above fog.
Inertia
The inertia speed point of an emulsion is determined on the Hurter and Driffield characteristic curve by the intercept between the gradient of the straight line part of the curve and the line representing the base + 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 𝜃 = 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. [2]
Historical systems
Warnerke
The first known practical sensitometer, which allowed measurements of the speed of photographic materials, was invented by the Polish engineer Leon Warnerke[3] – pseudonym of Władysław Małachowski (1837–1900) – in 1880, among the achievements for which he was awarded the Progress Medal of the Photographic Society of Great Britain in 1882.[4][5] 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 phosphorescent tablet excited before by the light of a burning magnesium ribbon.[5] 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.
His system saw some success but proved to be unreliable[3] 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.[5] The concept, however, was later built upon in 1900 by Henry Chapman Jones (1855–1932) in the development of his plate tester and modified speed system.[5][6]
Hurter & Driffield
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 H&D would require ten times the exposure of an emulsion rated at 2500 H&D.[7]
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 10". The H&D system was officially[8] accepted as a standard in the former Soviet Union from 1928 until September 1951, when it was superseded by GOST 2817–50.
Scheiner
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° Sch. to 20° 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° Sch. to 20° Sch.) corresponded to a hundredfold increase in sensitivity. Thus emulsions that differed by 1° Sch. on the Scheiner scale were -fold more (or, less) sensitive to each other. An increment of 3° Sch. came close to a doubling of sensitivity[7][9] .
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)[3] and Flemish-born botanist Walter Hecht (1896–1960), (who, in 1919/1920, jointly developed their Eder–Hecht neutral wedge sensitometer measuring emulsion speeds in Eder–Hecht grades). Still, it remained difficult for manufacturers to reliably determine film speeds, often only by comparing with competing products,[3] 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.[3][10]
Scheiner's system was eventually abandoned in Germany, when the standardized DIN system was introduced in 1934. In various forms, it continued to be in widespread use in other countries for some time.
DIN
The DIN system, officially DIN standard 4512 by the Deutsches Institut für Normung (then known as the Deutscher Normenausschuß (DNA)), was published in January 1934. It grew out of drafts for a standardized method of sensitometry put forward by the Deutscher Normenausschuß für Phototechnik[10] as proposed by the committee for sensitometry of the Deutsche Gesellschaft für photographische Forschung[11] since 1930[12][13] and presented by Robert Luther[13][14] (1868–1945) and Emanuel Goldberg[14] (1881–1970) at the influential VIII. International Congress of Photography (German: Internationaler Kongreß für wissenschaftliche und angewandte Photographie) held in Dresden from 3 to 8 August 1931.[10][15]
The DIN system was inspired by Scheiner's system,[3] but the sensitivities were represented as the base 10 logarithm of the sensitivity multiplied by 10, similar to decibels. 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...):[9]
As in the Scheiner system, speeds were expressed in 'degrees'. Originally the sensitivity was written as a fraction with 'tenths' (for example "18/10° DIN"),[16] where the resultant value 1.8 represented the relative base 10 logarithm of the speed. 'Tenths' were later abandoned with DIN 4512:1957-11, and the example above would be written as "18° DIN".[7] The degree symbol was finally dropped with DIN 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 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" would now be labeled as "21 DIN" without emulsion changes.
Originally only meant for black-and-white negative film, the system was later extended and regrouped into nine parts, including DIN 4512-1:1971-04 for black-and-white negative film, DIN 4512-4:1977-06 for color reversal film and DIN 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 6:1974,[17] ISO 2240:1982,[18] and ISO 5800:1979[19] where the same sensitivity is written in linear and logarithmic form as "ISO 100/21°" (now again with degree symbol). These ISO standards were subsequently adopted by DIN as well. Finally, the latest DIN 4512 revisions were replaced by corresponding ISO standards, DIN 4512-1:1993-05 by DIN ISO 6:1996-02 in September 2000, DIN 4512-4:1985-08 by DIN ISO 2240:1998-06 and DIN 4512-5:1990-11 by DIN ISO 5800:1998-06 both in July 2002.
BSI
When BS 935:1941 was published during World War II, specifying exposure tables for negative materials, it employed the same fixed-density speed criterion used in the German DIN 4512:1934 system. The British Standard also used logarithmic speed numbers, following the example of Scheiner and DIN. When the American ASA Z38.2.1:1943 standard was published, it used a fractional gradient speed criterion and arithmetic speed numbers, for compatability with Weston and GE. [20]
British standard BS 1380:1947 adopted the fractional gradient criterion of the American 1943 standard, and also included arithmetic speed numbers in addition to logarithmic numbers.[21] The logarithmic speed number proposed in the later BS 1380:1957 standard was almost identical to the DIN 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 degrees. [22]
Following an increasing effort to produce international standards, the British, American and German standards became identical in ISO 6:1974, which corresponded to BS 1380:Part1:1973. [23]
Weston
Before the advent of the ASA system, the system of Weston film speed ratings was introduced by Edward Faraday Weston (1878–1971) and his father Dr. Edward Weston (1850–1936), a British-born electrical engineer, industrialist and founder of the US-based Weston Electrical Instrument Corporation,[24] 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.,[25][26] who worked for them[27] and later received a Howard N. Potts Medal for his contributions to engineering.
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[28] 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.[29]
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,[30] 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.[30] 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; however the changes of the ASA PH2.5-1960 revision may be taken into account when comparing with newer ASA or ISO values.
General Electric
Prior to the establishment of the ASA scale[31] and similar to 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.
Film speed values for use with their meters were published in regularly updated General Electric Film Values[32] leaflets and in the General Electric Photo Data Book.[33]
General Electric switched to use the 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.[32][34] The company continued to publish recommended film values after that date, however, they were then aligned to the ASA scale.
ASA
Based on earlier research work by Loyd Ancile Jones (1884–1954) of Kodak and inspired by the systems of Weston film speed ratings[30] and General Electric film values,[32] 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 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: Exposure Index (EI).)
The ASA scale is a linear scale, that is, a film denoted as having a film speed of 200 ASA is twice as fast as a film with 100 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 HP3 that had been rated at 200 ASA before 1960 was labeled 400 ASA afterwards without any change to the emulsion. Similar changes were applied to the 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 speed value Sv (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 IT2.5–1986 of the National Association of Photographic Manufacturers, which represented the US adoption of the international standard ISO 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 IT2.21 in 1994, the US adoption of the ISO 2240 standard.
On an international level, the ASA system was superseded by the 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
GOST (Cyrillic: ГОСТ) was an arithmetic film speed scale defined in GOST 2817-45 and GOST 2817–50.[35][36] It was used in the former Soviet Union since October 1951,[citation needed] replacing Hurter & Driffield (H&D, Cyrillic: ХиД) numbers,[35] which had been used since 1928.[citation needed]
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 0.1.[37] GOST markings are only found on pre-1987 photographic equipment (film, cameras, lightmeters, etc.) of Soviet Union manufacture.[38]
On 1 January 1987, the GOST scale was realigned to the ISO scale with GOST 10691–84,[39]
This evolved into multiple parts including GOST 10691.6–88[40] and GOST 10691.5–88,[41] which both became functional on 1 January 1991.
Current system: ISO
The ASA and DIN film speed standards have been combined into the ISO standards since 1974.
The current International Standard for measuring the speed of color negative film is ISO 5800:2001[19] (first published in 1979, revised in November 1987) from the International Organization for Standardization (ISO). Related standards ISO 6:1993[17] (first published in 1974) and ISO 2240:2003[18] (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 with digital still-cameras is described in ISO 12232:2019 (first published in August 1998, revised in April 2006, corrected in October 2006 and again revised in February 2019).[42][43]
The ISO system defines both an arithmetic and a logarithmic scale.[44] 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 200/24° is twice as sensitive as one rated ISO 100/21°.[44]
Commonly, the logarithmic speed is omitted; for example, "ISO 100" denotes "ISO 100/21°",[45] while logarithmic ISO speeds are written as "ISO 21°" as per the standard.
Conversion between current scalesedit
Conversion from arithmetic speed S to logarithmic speed S° is given by[17]
and rounding to the nearest integer; the log is base 10. Conversion from logarithmic speed to arithmetic speed is given by[46]
and rounding to the nearest standard arithmetic speed in Table 1 below.
| APEX Sv (1960–) | ISO (1974–) arith./log.° |
Camera mfrs. (2009–) | ASA (1960–1987) arith. |
DIN (1961–2002) log. |
GOST (1951–1986) arith. |
Example of film stock with this nominal speed |
|---|---|---|---|---|---|---|
| −2 | 0.8/0°[47] | 0.8 | 0[48] | FPP BW Super Positive[49] | ||
| 1/1° | 1 | 1 | (1) | Svema Micrat-orto, Astrum Micrat-orto | ||
| 1.2/2° | 1.2 | 2 | (1) | |||
| −1 | 1.6/3° | 1.6 | 3 | 1.4 | ||
| 2/4° | 2 | 4 | (2) | |||
| 2.5/5° | 2.5 | 5 | (2) | |||
| 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 | ||
| 1 | 6/9° | 6 | 9 | 5.5 | original Kodachrome | |
| 8/10° | 8 | 10 | (8) | Polaroid PolaBlue | ||
| 10/11° | 10 | 11 | (8) | Kodachrome 8 mm film | ||
| 2 | 12/12° | 12 | 12 | 11 | Gevacolor 8 mm reversal film, later Agfa Dia-Direct | |
| 16/13° | 16 | 13 | (16) | Agfacolor 8 mm reversal film | ||
| 20/14° | 20 | 14 | (16) | Adox CMS 20 | ||
| 3 | 25/15° | 25 | 15 | 22 | old Agfacolor, Kodachrome II and (later) Kodachrome 25, Efke 25 | |
| 32/16° | 32 | Zdroj:https://en.wikipedia.org?pojem=Film_speed