Photography film and sensor speed equivalents
Before ISO film-speed notation became standard, two of the most widely encountered systems were the American ASA scale and the German DIN scale. ASA is an arithmetic scale: a doubling of the ASA number represents a one-stop increase in film sensitivity. Thus ASA 100, 200, 400 and 800 each represent one stop more sensitivity than the preceding value.
DIN is a logarithmic scale, expressed in degrees (°). An increase of 3° DIN represents one f-stop. Thus DIN 21, 24, 27 and 30 correspond approximately to ASA 100, 200, 400 and 800 respectively. In other words, the two systems describe the same thing in different ways.
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ISO
The ISO standard brought the two approaches together. Film speed is properly expressed as ISO 100/21°, This combines the arithmetic ASA-style value with the logarithmic DIN value. Unfortunately however, the arithmetic ASA component became by far the more commonly used part. Photographers generally say “ISO 100”, “ISO 400” or “ISO 1600”, rather than “ISO 100/21°”, even though the latter is the correct ISO designation.
Which is a pity because there is something particularly useful about the DIN scale for photographers. Its logarithmic nature maps directly onto exposure stops. Every increase of 3° DIN represents a doubling of sensitivity. In other words, a change of 3° DIN is exactly one f-stop.
On the ASA scale, the arithmetic number doubles. Problem with that is that ASA numbers inflate dramatically, perhaps giving the impression the camera is more sensitive than it really is. Whereas the actual photographic capability is more meaningfully represented by the DIN number. So although the ASA number became the familiar everyday shorthand, the DIN number arguably gives the more elegant and representative description of the relationship between film speed and exposure.
More photographic utilities
- You might also be interested in our Photographic Neutral Density Filter Equivalents table;
https://garfnet.org.uk/tables/general/photography-neutral-density-filters - Or the handy Depth of field Calculator applet
https://garfnet.org.uk/dof