DIN system of 'film speed', a common but wrong statement

xpatUSA
Aug. 28, 2026
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    Members 1561 posts
    Aug. 28, 2026, 2:19 p.m.
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    It is often said that 3-degree steps in the DIN scale represents a doubling of halving of sensitivity, sometimes even using the word "exactly" ...

    But the 1-degree step ratio of the said scale is chosen such that ten steps represent exactly ten times or one tenth of the sensitivity. That makes the DIN ratio equal to the 10th root of 10, not the cube root of 2 as found in ISO/ASA.

    10th root of 10 = 1.25892541 according to my Casio, and
    cube root of 2 = 1.25992105.

    So a doubling on the DIN theoretical scale is a factor of 1.995, not exactly 2.000.

    Before anyone rushes to ask "what does that matter?!!" I do realize that the ratios are close enough for government work ...

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    Team
    Aug. 28, 2026, 3:24 p.m.
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    It is like dB (decibel) - 3dB is said to be 'double whatever', but in reality one bel is 10 times and decibel is 10th root of 10.
    For ten steps DIN (or 10dB) (about 10.08 times, if calculated based on cube root of 2) error is 0.8 percent - close enough for everybody (except for space missions).