International Organization for Standardization. (yes, it’s not IOS) The Swiss-based international agency responsible for coordinating every kind of international standard imaginable, including film speed standards. Most exposure indices or film speeds today are described by the ISO system, which uses the same numeric values as the old ASA system. (eg: ISO 100, 400, 800) The slower the film the lower the number. By today’s standards ISO 100 is slow film and ISO 800 or 1600 is fast film. Though it should be noted that these speeds represent technical advances in film technology - as recently as a couple of decades ago, 64 was slow and 200 or 400 was thought of as fairly fast.
Technically the ISO standard seems to list both the ASA-style film speed and the DIN-style film speed (eg: ISO 100/21°) which is an informationally redundant compromise. To the undoubted dismay of the Germans, however, hardly anyone uses the DIN numbers anymore.
Digital cameras do not use film and so cannot have film speed ratings as such. However, many digital cameras have adjustable light sensitivity levels, and these adjustable levels are stated as ISO film speed equivalents, simply because the ISO film speed numbers are a well-understood and handy convention.
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