Film speed
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 United States 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, 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 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
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, inertia, fixed density, minimum useful gradient and [|fractional gradient].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 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.Minimum useful gradient
The minimum useful gradient criterion places the speed point where the gradient first reaches an agreed value.Fractional gradient
The fractional gradient is defined as the speed point at which the slope of the characteristic curve first reaches a fixed fraction of the average gradient over a range 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 – pseudonym of Władysław Małachowski – in 1880, among the achievements for which he was awarded the Progress Medal of the Photographic Society of Great Britain in 1882. 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. The speed of the emulsion was then expressed in 'degrees' Warnerke 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 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. The concept, however, was later built upon in 1900 by Henry Chapman Jones in the development of his plate tester and modified speed system.
Hurter & Driffield
Another early practical system for measuring the sensitivity of an emulsion was that of Hurter and Driffield, originally described in 1890, by the Swiss-born Ferdinand Hurter and British Vero Charles Driffield. 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.The methods to determine the sensitivity were later modified in 1925 and in 1928 —this later variant was sometimes called "H&D 10". The H&D system was officially 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 system was devised by the German astronomer Julius Scheiner 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° 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. corresponded to a hundredfold increase in sensitivity. Thus emulsions that differed by 1° Sch. on the Scheiner scale were -fold more sensitive to each other. An increment of 3° Sch. came close to a doubling of sensitivity.The system was later extended to cover larger ranges and some of its practical shortcomings were addressed by the Austrian scientist Josef Maria Eder and Flemish-born botanist ,. It remained difficult for manufacturers to reliably determine film speeds, often only by comparing with competing products, 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.
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, was published in January 1934. It grew out of drafts for a standardized method of sensitometry put forward by the Deutscher Normenausschuß für Phototechnik as proposed by the committee for sensitometry of the Deutsche Gesellschaft für photographische Forschung since 1930 and presented by and Emanuel Goldberg at the influential VIII. International Congress of Photography held in Dresden from 3 to 8 August 1931.The DIN system was inspired by Scheiner's system, but the sensitivities were represented as the base 10 logarithm of the sensitivity multiplied by 10, similar to decibels. Thus an increase of 20° represented a hundredfold increase in sensitivity, and a difference of 3° was much closer to the base 10 logarithm of 2 :
As in the Scheiner system, speeds were expressed in 'degrees'. Originally the sensitivity was written as a fraction with 'tenths', 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". 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, ISO 2240:1982, and ISO 5800:1979 where the same sensitivity is written in linear and logarithmic form as "ISO 100/21°". 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 compatibility with Weston and GE.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.