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Posted by Cliff Smith on February 05, 2002 at 06:25:38:
So shutter speed is a measure of time in fractions of a second, and the f-stop number is derived from a ratio of the focal length to the aperture, but what about ISO? I know what ISO stands for, but how is the number derived? When I set an ISO of 100, what am I getting 100 of?
Posted by John Ayres on February 05, 2002 at 07:35:50:
To measure each film's sensitivity to light, a system of numbers called ISO (International Standards Organization) was devised. Formerly called ASA, these numbers denote film speed, most commonly from very "slow" low ISO films to the "ultrafast" high ISO films. Although the ISO series runs from 6 to 6400, the most common film speeds are ISO 25, 50, 100, 200, 400, 800, 1600, and 3200. There are also films of intermediate ISO, especially ISO 64, 160, and 1000. It's important to remember that as the ISO number doubles, the film is twice as sensitive to light. Thus ISO 200 film would require half as much light as ISO 100 film.
If you use a higher ISO speed film for enlargements, more grain will show up in your photo. If you plan to enlarge or crop your photos, it would be a good idea to use a slower film if you have enough light. Sometimes you have to compromise.
Posted by Winfried on February 05, 2002 at 12:49:43:
Determining a film's sensitivity to light is not a very easy task. There are worldwide recognized standards on how to 'measure' it. Actually a film's 'speed' can't be measured directly but has to be evaluated with lab experiments and measuring the density of the exposed and processed film.
For color print and slide films there are standard processing methods and such a film will very probably have the same speed rating whereever you have it processed.
With b/w film it is different. The film speed is determined using a 'standard' processing method but with special film developers you might get other results.
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