IF YOU'RE a photographer with more than one or two lenses in your bag, the time's a-comin' when you'll start thinking about purchasing a second camera body to attach them to.

A second body can work wonders. For openers, it will enable you to shoot two types of film without the hassle of rewinding one roll and loading another. Until the day camera manufacturers decide that interchangeable film backs (a key feature of roll-film SLRs such as the reknowned Hasselblad) are potential big sellers on the 35mm market, a second body is your only alternative.

And though a single body with a zoom lens is a miraculous tool indeed, there are times when two bodies -- one with a wide-angle lens, one with a telephoto -- will bring home pictures you'd have missed otherwise. Fixed focal-length optics are faster than zooms, allowing you to shoot in available gloom. And a zoom that will take you from 28mm to 200mm with one twist of the lens collar is still a gleam in lens designers' eyes.

Finally, a second body will give you a backup when your first SLR jams or falls victim to dead batteries.

So, what do you look for? If you're reasonably happy with the lenses you've got now, by all means stick with the manufacturer of your first body; that way, the optics in your bag will work with both. That doesn't necessarily mean, however, that you should buy the same model. Instead, consider the second camera as a way to add to your photographic versatility.

Before you go looking at hardware, give careful thought to your current photographic strengths and weaknesses and evaluate how your current camera body relates to them.

The best way to do that is to haul out your photo collection -- a dozen or so randomly selected boxes of slides, or a like number of prints -- and critique them. Sort the pictures into general subject categories; examine the time of day and types of lighting you shoot the most. Think, too, about the types of activities -- hiking, travel, family gatherings, neighborhood walks -- your photography's tied into. Final your pictures' technical quality. Are a great many shots badly exposed or out of focus?

Once you've taken stock, you're in a position to pick the camera you need. For example, if your present body is a match-needle, manual- exposure SLR, and you find a high proportion of sports and action pictures in your "take," you ought to look at quicker-handling auto- exposure models, perhaps one with a built-in power winder or an action- stopping 1/2,000 shutter speed.

If you discover that many of your pictures are taken at twilight, perhaps you should examine a model with an extended range of slow shutter speeds, a meter that reads light off the film plane (to fine-tune exposure during long exposures in rapidly changing light) and a brightly lit viewfinder so you can see what you're doing in the gloom.

If your pictures are predominantly taken at parties or family gatherings, you may decide to sacrifice lens interchangeability for a camera you can tuck away in your pocket. "Clamshell cameras" -- so-called because of the curved covering which slides over the lens for protection -- usually boast moderate wide-angle lenses and built-in flash, making them great for small-group pictures at parties.

Finally, if your pictures are often technically flawed, you should look toward automatic-exposure and self- focusing cameras as a potential cure for what ails them.

Before you write off the cost of a second body as an extravagance, keep in mind that there's a wide variety of used cameras that will fit the bill. Saturday and Sunday classified ads are a treasure trove for used gear; many area camera stores (particularly the larger ones) accept trade-ins on new merchandise, which they sell at surprisingly affordable prices. Local camera clubs, too, are a good source. If you strike out locally, check into Shutterbug Ads (P.O. Box F120, Titusville, FL 32781), a monthly national listing of merchandise buyers and sellers.