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 100 = DIN 21
ISO 200 = DIN 24
ISO 400 = DIN 27
ISO 800 = DIN 30
This information is really just to quench your curiosity. If you're using a film camera with a light meter, you already presumably figured out that ISO and ASA are interchangeable.
Very little film is being made anyhow. But if you do stumble upon films like ORWO NP15, NP21, or NP27 you now understand that the film speed is in the title!
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