If you use an old film camera you may have noticed there's a dial with the abbreviations ASA and right next to it DIN. You likely have already figured out to use ASA where your film says ISO. ISO and ASA are interchangeable. From prehistoric times (ok, since the middle of the 20th century) to the late 1980's ASA was what much of the world used as measure of film sensitivity. The abbreviation stood for American Standards Association. At the same time, the German version of the organization that measured film sensitivity was abbreviated DIN or Deutsches Institut für Normung meaning German institute for Standards. Germany was and to some degree still is an influential player in photographic innovation and manufacture. DIN wasn't a linear measure. Doubling of sensitivity was shown by adding 3 to the number. For example 21 DIN (=100 ASA/ISO) and 24 DIN (=200 ASA/ISO.) DIN and ASA ISO are equal at 12. So: ISO 12 = DIN 12 ISO 25 = DIN 15 ISO 50 = DIN 18 ISO 10
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