If you shoot color print film, bracketing is probably not necessary. But if you shoot slide film, it has such a narrow tolerance for exposure errors, that it is a good idea to take several shots of your composition at the exposure recommended by your meter, and then take several more images at higher and lower exposure settings--"bracketing" your primary exposure to be safe.

For example, the recommended exposure for the photo of the barn above (#1) was f/16 and 1/60th of a second. To bracket, you would also take a second shot at f/16 and 1/125th of a second (#2), and a third shot at f/16 and 1/30th of a second (#3). Why not leave the shutter speed at 1/60th and reshoot at f/8 and f/22? You could, but in outdoor photography, you usually want to maintain depth of field, and decreasing the f/ stop will affect depth of field. You also stand a good chance of moving the focusing ring as you change f/stops, and could end up with an out-of-focus picture.

With today's sophisticated in-camera systems, you will have a higher percentage of pictures that are on the mark at the recommended exposure, as in the three examples above. And sometimes it seems a waste of film and money to bracket. But there are three good reasons to bracket anyway. First is for insurance. If this is the shot of a lifetime, why take a chance on a poorly exposed image? Second, your camera's meter will get fooled now and then. No meter, no matter how expensive the camera, can handle every lighting situation. Third, technically correct exposure does not always produce the best image. Sometimes one of the bracketed shots is a better shot from a creative point-of-view, especially when bracketing at 1/3 or 1/2 stops instead of full stops. When you compare your slides on the light table, you may find that you prefer a slide that is not at the "recommended" exposure.

For more precise bracketing, you can use the exposure compensation control on your camera, if it has one, to bracket in 1/3 or 1/2 stops. Some cameras also have optional multi-function backs that allow bracketing in smaller increments, and some cameras even come with on-board bracketing options that allow you to select a specific bracketing program.

Bracketing is a good practice that increases your chances of having a good shot when using slide film. It is not unusual for one of your bracketed shots to actually be a better image than the recommended exposure.