asa versus iso

Started Oct 5, 2005 | Discussions thread
Michael J. Hußmann Senior Member • Posts: 1,293
Re: asa versus iso

In the beginning, sensitivity was either specified according to the US norm (ASA) or the German DIN norm. ASA was linear and DIN logarithmic, i.e. 100 ASA = DIN 21, 200 ASA = DIN 24, 400 ASA = DIN 27 – doubling the ASA figure was equivalent to adding 3 to the value according to DIN. Several years ago, ISO, in poorly conceived attempt tried to reconcile both measures, suggested that both values should be combined: ISO 100/21, ISO 200/24 etc.. After a while, only the ASA value survived, but with an "ISO" prefix.

In other words: x ASA = ISO x.

  • Michael

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