Quote Originally Posted by Peltigera View Post
It is not quite right to say that ASA was arithmetic and DIN was logarithmic (although that is correct) as the DIN system used (uses) a very different emulsion characteristic to measure film speed than the ASA system did. A large ASA difference in film speed between two films would not necessarily mean a large DIN difference. The DIN system (or degree part of ISO) measures a fixed density above the fog level while the ASA system ( and first part of ISO) is based on the gradient of the log exposure/density curve. It is at least theoretically possible for one film to be faster than another according to DIN and slower according to ASA.
It is my understanding that when the ISO standard was put in place, the ASA and DIN procedures for determining sensitivity were replaced with a single, ISO procedure.