Q. Can you tell me about the "gray market" in camera sales? Is this a good deal for people like me who need to save a dollar or two but still want to keep active in photography? Is this stuff legal? Moral?

A. Gray-market camera products are not made to be sold in the U.S. consumer market. They are sold in foreign shops, and frequently at airports (sometimes in duty-free shops) or in such places as post exchanges overseas.

What happens is that someone gets hold of a group of brand-name products at such places and brings them to the U.S. for sale.

This is not illegal!

The problem is that usually the equipment, while it generally is less expensive, carries no guarantee of service and maintenance in the United States. It may have to be sent back to whoever sold it to you for service. He may then have to send it back to Japan or elsewhere. This could take a long time.

Gray market stuff is hard to come by in this area. Most of the better camera stores that I am familiar with won't mess with it, and prefer to handle equipment that carries the proper guarantees and quality control for the American market. FILM LIFE Q. I've been told that it's a bad idea to leave exposed film in the camera or to let it sit around for any length of time. This presents a problem. Suppose I buy a fresh roll of film and shoot about half of it at a party or at school? I can't just take it out and waste the rest! Even if I shoot the whole thing, I might not get around to getting it developed for a couple of weeks.

Last year we were away for four weeks on vacation. I shot 12 rolls of color and two rolls of black-and-white. We didn't get them all processed for over a month. Everything seemed OK, and I only missed about one out of 15 shots. Am I worrying for nothing?

A. I'm glad your film came out OK. You may have been a little lucky. In such circumstances, there are a couple of things you should be careful of.

First of all, take care of your film. If you buy lots of rolls, be sure to store them in the refrigerator, or at least somewhere cool and dry. Also be sure that the film is fresh and not near its expiration date.

Never, never carry film around in the glove compartment or trunk of the car for more than very brief periods.

Try very hard to use the roll up before you put it away for the rest of the summer. If you let it lie around waiting to shoot again, you can damage the images already made: Half a roll is better than none. And besides, you generally only have to pay the processing costs for what you've shot.

Both Kodak and Fuji sell mailers that can be very valuable. Even if you're traveling, you can send your film off for processing, and it'll be there when you get home. DOUBLE EXPOSURES

Q. I just saw a picture of the Tomb of the Unknowns at Arlington Cemetery with an American Flag superimposed on it. It was beautiful, and when I asked the photographer how he did it, he said it was a double exposure. I tried to look knowledgable but walked away wondering how I can make an intentional double exposure.

A. You can make double exposures, or even triple or more exposures and have a good time doing so.

Many of the new cameras have a double exposure button or control that makes it easy. If so, your owner's manual will tell you how. If not, the way to do it is this:

Before your first exposure in a multiple series, gently twist your re-wind knob until the film is tight. Make your exposure, then use the re-wind knob to gently tighten the film again.

Then push your re-wind button in to release the film transport. Hold the film re-wind knob firmly and cock your shutter (usually with the thumb lever).

Then shoot again.

Remember, whether the double exposure button is built in or if you're setting up the double manually, be careful of your exposure. If you're doing two shots, divide the correct exposure in half. That is, if the correct exposure is 1/250 at f.8, shoot both of the exposures at 1/500 at f.8.

This is all chancey without a built-in double exposure device on your camera. Unless the film transport mechanism is totally disengaged, your lineup can be off enough to spoil the special effect you're seeking. So when you try this, overshoot by plenty.