ASA is a scale for film speed, which means how sensitive a film is to light. It comes from the American Standards Association, which published its film speed standard in 1943. On the ASA scale, doubling the number means the film needs half as much light, for example ASA 100 to ASA 200.
In 1974 the international ISO standard combined the ASA and German DIN scales. During the 1980s, ISO replaced ASA on film boxes and cameras. ISO 100 uses the same number as ASA 100. The American Standards Association itself later became ANSI.
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