*the ASA and DIN film speed standards have been combined into the ISO standards since 1974*
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The current International Standard for measuring the speed of colour negative film is ISO 5800:2001[17] (first published in 1979, revised in November 1987) from the International Organization for Standardization (ISO).
Related standards ISO 6:1993[15] (first published in 1974) and ISO 2240:2003[16] (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 colour 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.[38]
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°.[38]
Commonly, the logarithmic speed is omitted; for example, “ISO 100” denotes “ISO 100/21°”,[39] while logarithmic ISO speeds are written as “ISO 21°” as per the standard.
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