groping in the dark

ISO - Whatever That Stands For

Actually, it stands for International Standard Organization, the same guys who certify the companies who proudly display the "ISO 9000 Certified!" banners you see when you drive through a manufacturing district. What it means w/r/t film is the rating of a film's sensitivity to light or its film speed. Sometimes you see ANSI or ASA numbers for film; it's the same thing.

Most commercial film you see these days is either 100, 200 or 400 ISO. The higher the ISO number, the "faster" the film, i.e., shutter speeds will need to be faster to produce the same exposure under the same lighting conditions. For this reason, faster films are used to capture objects in motion, such as hummingbirds or race cars. Fast shutter speeds can "freeze" a hummingbird's wings, which would otherwise be a blur on slower film at slower shutter speeds. Note too, that that higher numbered films require less light to achieve an image. That's why the 100 speed film is often labelled "daylight" or "bright sun" while 400 speed film is recommended for indoor use.

Photography quirk-of-the-day:

Film is actually available in a wide variety of different speeds from 25 to 3200 in increments that correspond to 1/3 of a camera's f-stop. For reasons that require some math (which I'm not prepared to go into right now...wait until I'm done reading Everything & More), these increments are not evenly spaced, they increase geometrically: 50, 64, 80, 100, 125, 160, 200, 250, 320, 400, 500, 640, 800, etc. Under similar lighting conditions, 100 ISO film would reuire one stop down to achieve the same exposure using 50 ISO film. 200 ISO would be 2 stops, 400 ISO would be 3 stops, etc.

You might think that you should just go out and buy the fastest film you can so that you will have enough latitude to shoot in dim light and reduce blurring. Unfortunately, like everything in photography, there is a tradeoff. In order to produce an image in low light conditions, the silver crystals (or grain) in film are bigger. In the fastest films, this can be distracting and you can lose a lot of fine detail in the "fuzziness" that grain produces. Slower speed film with its fine grain structure and increased contrast produce much sharper and detailed images.

Some newer technologies can reduce the appearance of grain in faster films, but as a rule of thumb, the slower the film the finer the grain. The general wisdom is that 400 speed film gives you the most latitude with a somewhat acceptable degree of detail, at least for prints at 8x10 inches or less. Right now, I'm using Kodak Plus-X 125 and Kodak Tri-X 400 depending on my lighting conditions.

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