For something to do on long evenings besides take another picture of the cat, sit down and evaluate your photography.

First, collect all the slides and prints you can find from the last year or so. Look over the sampling. Do you find yourself saying, "Gee, that would have been a nice shot if only I'd . . ."? If so, begin sorting those "if onlies" according to types.

For example, maybe you have a large stack of "if only they'd been sharper" shots. That could mean you failed to focus properly or moved the camera while shooting; or the subject moved as you shot with a slow shutter speed, the subject was too close to the lens (especially with cheaper "fixed focus" cameras), or the camera is broken.

"If only the background had been better" is easy to diagnose, harder to fix. The entire scene must be taken in before the shutter is snapped. If the background is too busy and distracts from the subject, change camera position or subject location.

"If only I'd been closer" is another common digression from effective photography. People-shots taken from too far away are often the result of timid photographers. If you have trouble asserting yourself photographically, remember: most people enjoy having their photographs taken -- especially when the results are flattering. And film isn't cheap -- in fact, it's getting more expensive by the month.

"If only the light had been better" is another common malady that can be remedied with forethought. And good, adjustable camera combined with fairly fast film (ASA 200 or higher) can make an exposure, even in low light. But may not be a good exposure.

There should be enough available light to offer a marked contrast between lightest and darkest areas of the scene (except, of course, in the case of night shots or fog shots).

If many shots seem to suffer from poor lighting, perhaps you'd better invest in a good electronic flash unit. Buy one with automatic operation for easiest use. An energy-saving thyristor circuitry will conserve on battery costs.

"If only I hadn't chopped the head (feet) off" indicates sloppy photographic habits. You're not taking the time to view properly. Or, if you're shooting a camera other than a single lens reflex type, you're not observing those corner frame lines in the viewfinder.

Those lines indicate where the corners of the actual image will fall. The area visible beyond the frame lines will not appear on film. Keep that in mind the next time you pick up your 110 cartridge, instantprint, rangefinder or viewfinder camera.

"If only I'd taken some photos of this, that or the other" indicates an obvious desire to have more shots of those things that mean a lot to you. The solution: Hire a professional photographer at $500 a day plus expenses or, for the price of a little more film, do the job yourself. Remember, you can always take your camera along -- even if you don't use it. But you can't leave your camera home and take even a single photograph.

Once you've analyzed your photographic "crimes" and made plans to "go straight," stop and think for a few more minutes. What are you going to do with all those slides or prints lying on the living room floor?

If the answer is a vague "I don't know," you're guilty of one more crime against photography, perhaps the most dastardly of all: not adequately storing your shots. Either buy enough slide trays or print albums to hold the most valuable shots or be resigned to not being able to locate what you want when you want it.

Q: I would like to sell a Strobo Flash II and a Teleflash unit, each used just a few times. The Strobo cost about $116 and the Teleflash $50 when new. Can you tell me their value and how I can sell them?

A: With the price of electronic flash units coming down almost yearly, you'll probably do well to get half of what you paid. You can sell them through a classified ad in the newspaper under the category "Photographic Equipment," or in Shutterbug Ads, the nationwide marketplace for photographic gear, P.O. Box F, Titusville, Fla. 32780 (single-issue price $2).