ABSTRACT
Film manufacturers have spent a lot of time and resources establishing the film speed and the development time suggestions for their products. Not knowing the exact combination of products we use for our photographic intent, they have had to make a few assumptions. These assumptions have led to an agreement among film manufacturers, which were published as a standard in ASA PH2.5–1960. It was the first standard to gain worldwide acceptance, but it went through several revisions and was eventually replaced by the current standard ISO 6:1993, which combines the old ASA geometric sequence (50, 64, 80, 100, 125, 160, 200, …) with the old DIN log sequence (18, 19, 20, 21, 22, 23, 24, …). As an example, an ISO speed is written as ISO 100/21°.
