"
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
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).
[36][37]
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"
Photography — Determination of ISO speed of monochrome (black-and-white), continuous-tone photographic negative materials for still photography