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H&D system was officially accepted as a standard in the former Soviet Union from 1928 until September 1951, when it was superseded by GOST 2817–50. 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". For example, an emulsion rated at 250 H&D would require ten times the exposure of an emulsion rated at 2500 H&D. In their system, speed numbers were inversely proportional to the exposure required. Hurter & Driffield Īnother 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). 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. 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. Each number represented an increase of 1/3 in speed, typical plate speeds were between 10° and 25° Warnerke at the time. or °W.) corresponding with the last number visible on the exposed plate after development and fixation. The speed of the emulsion was then expressed in 'degrees' Warnerke (sometimes seen as Warn. 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 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 (1837–1900) – in 1880, among the achievements for which he was awarded the Progress Medal of the Photographic Society of Great Britain in 1882. To determine the exposure time needed for a given film the light meter was usually used.įilm speed measurement systems Historical systems Warnerke Ultimately sensitivity is limited by the quantum efficiency of the film or sensor. In short, the higher the sensitivity, the grainier the image will be. 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 of other types). Exposure times for early photgraphic emulsions went from hours to minutes and then seconds and fractions of a second in the 1800's. Emulsions that were less sensitive were called slower because the time to complete an exposure was much longer and often usable only for still lifes. 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 their pose and not make any body movements for a shorter length of time. The term speed comes from the early days of photography. Prior to ISO gaining traction the best known systems were ASA in the U.S. A closely related ISO system is used to describe the relationship between exposure and output image lightness in digital cameras. (As is common, the "100" in the film name alludes to its ISO rating.)įilm 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 the mid-1970s. The second is often dropped, making (e.g.) "ISO 100" effectively equivalent to the older ASA speed. This film container denotes its speed as ISO 100/21°, including both arithmetic (100 ASA) and logarithmic (21 DIN) components.
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