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).
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 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 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.
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 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. 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. 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.
Weston from about 1935 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,
Edward Weston (1850–1936), a British-born electrical engineer, industrialist and founder of the US-based
Weston Electrical Instrument Corporation, 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., who worked for them 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 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. 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, 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. 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 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 leaflets and in the
General Electric Photo Data Book. 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. 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 and
General Electric 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 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 film, with a sensitivity of 65
GOST (ГОСТ) (Cyrillic: ) was an arithmetic film speed scale defined in GOST 2817-45 and GOST 2817–50. It was used in the former
Soviet Union from October 1951, replacing Hurter & Driffield (H&D, Cyrillic: ХиД) numbers, which had been used since 1928. 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. GOST markings are only found on pre-1987 photographic equipment (film, cameras,
lightmeters, etc.) of Soviet Union manufacture. On 1 January 1987, the GOST scale was realigned to the
ISO scale with GOST 10691–84, This evolved into multiple parts including GOST 10691.6–88 and GOST 10691.5–88, 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 (first published in 1979, revised in November 1987) from the
International Organization for Standardization (ISO). Related standards ISO 6:1993 (first published in 1974) and ISO 2240:2003 (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). The ISO system defines both an
arithmetic and a
logarithmic scale. 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°. Commonly, the logarithmic speed is omitted; for example, "ISO 100" denotes "ISO 100/21°", while logarithmic ISO speeds are written as "ISO 21°" as per the standard.
Conversion between current scales FR with both ASA and DIN markings Conversion from arithmetic speed
S to logarithmic speed
S° is given by :S^\circ = 10 \log S + 1 and rounding to the nearest integer; the log is base 10. Conversion from logarithmic speed to arithmetic speed is given by :S = 10^{\left( {S^\circ - 1} \right)/10} and rounding to the nearest standard arithmetic speed in Table 1 below. 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
Sv values 1 to 10 correspond with logarithmic ASA grades 1° to 10° found in ASA PH2.5-1960. • ASA arithmetic speeds from 4 to 5 are taken from ANSI PH2.21-1979 (Table 1, p. 8). • ASA arithmetic speeds from 6 to 3200 are taken from ANSI PH2.5-1979 (Table 1, p. 5) and ANSI PH2.27-1979. • ISO arithmetic speeds from 4 to 3200 are taken from ISO 5800:1987 (Table "ISO speed scales", p. 4). • ISO arithmetic speeds from 6 to 10000 are taken from ISO 12232:1998 (Table 1, p. 9). • ISO 12232:1998 does not specify speeds greater than 10000. However, the upper limit for
Snoise 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. 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 12232:2019. • Most of the modern
35 mm film SLRs support an automatic film speed range from ISO 25/15° to 5000/38° with
DX-coded films, or ISO 6/9° to 6400/39° manually (without utilizing
exposure compensation). The film speed range with support for
TTL flash is smaller, typically ISO 12/12° to 3200/36° or less. • The Booster accessory for the
Canon Pellix QL (1965) and
Canon FT QL (1966) supported film speeds from 25 to 12800 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). On this camera exposure compensation and extreme film speeds were mutually exclusive. • The
Leica R8 (1996) and
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 0.8/0° to ISO 100000/51° in half exposure steps. • Digital camera manufacturers' arithmetic speeds from 12800 to 409600 are from specifications by Nikon (12800, 25600, 51200, 102400 in 2009, 204800 in 2012, 409600 in 2014), Canon (12800, 25600, 51200, 102400 in 2009, 204800 in 2011, 4000000 in 2015), Sony (12800 in 2009, 25600 in 2010, 409600 in 2014), Pentax (12800, 25600, 51200 in 2010, 102400, 204800 in 2014), and Fujifilm (12800 in 2011).
Historic ASA and DIN conversion 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 in which 21/10° DIN was converted to ASA 80 instead of ASA 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° DIN is related to ASA 80, 18° DIN to ASA 40, etc. Users of classic cameras may become confused if they are not aware of the historic background of changing standards.
Determining film speed 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
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
Hm, in
lux-s, is that for point m when the specified contrast condition is satisfied. The ISO arithmetic speed is determined from: :S = \frac {0.8\;\text{lx⋅s}} {H_\mathrm{m}} This value is then rounded to the nearest standard speed in Table 1 of ISO 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
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
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 allows twice as much light to strike the focal plane as a lens set to 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: 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 ISO film). This known as the
sunny 16 rule.
Exposure index 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 400 film at EI 800 and then use
push processing to obtain printable negatives in low-light conditions. The film has been exposed at EI 800. Another example occurs where a camera's
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==