How Photographic Film Works

­When you purchase a roll of film for your camera, you have a lot of choices. T­hose products that have the word "color" in their name are generally used to produce color prints that you can hold in your hand and view by reflected light. The negatives that are returned with your prints are the exposures that were made in your camera. Those products that have the word "chrome" in their name produce a color transparency (slides) that requires some form of projector for viewing. In this case, the returned slides are the actual film that was exposed in your camera.

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.