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Film comes with an ASA (American Standards Association) or ISO (International Standards Organization) rating that tells you its speed. The ISO and ASA scales are identical. Here are some of the most common film speeds:
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Once you decide on prints or slides, the next major decision is the film speed. Generally, the relative speed rating of the film is part of its name (MYColor Film 200, for example). ISO and ASA speed ratings are also generally printed somewhere on the box. The higher the number, the "faster" the film. "Faster" means increased light sensitivity. You want a faster film when you're photographing quickly moving objects and you want them to be in focus, or when you want to take a picture in dimly lit surroundings without the benefit of additional illumination (such as a flash).
When you make film faster, the trade-off is that the increased light sensitivity comes from the use of larger silver-halide grains. These larger grains can result in a blotchy or "grainy" appearance to the picture, especially if you plan to make enlargements from a 35-mm negative. Professional photographers may use a larger-format negative to reduce the degree of enlargement and the appearance of grain in their prints. The trade-off between photographic speed and graininess is an inherent part of conventional photography. Photographic-film manufacturers are constantly making improvements that result in faster films with less grain.
A slow-speed film is desirable for portrait photography, where you can control the lighting of the subject, the subject is stationary, and you are likely to want a large print from the negative. The finer silver-halide grains in such film produce the best results.
The advanced amateur photographer might encounter additional film designations such as tungsten balanced or daylight balanced. A tungsten-balanced film is meant to be used indoors where the primary source of light is from tungsten filament light bulbs. Since the visible illumination coming from a light bulb is different than from the sun (daylight), the spectral sensitivity of the film must be modified to produce a pleasing picture. This is most important when using a transparency film.
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