Q. I would welcome your opinion on the reflecting lenses that are on the market: 500mm and 1,000mm. I would use this type of lens for bird photography. Do you feel these lenses are worth the effort?

A. Reflecting lenses, similar to those used at astronomical observatories, are commonly called "mirror" lenses.

With a reflecting camera lens, the light enters the front and is bounced back and forth internally so that the physical size and weight of the lens is dramatically reduced while its focal length is increased.

In a mirror lens the front glass can be flat or curved. In either case you can see a circle in the middle of it, which is a mirror mounted near the back of the lens. Light comes through the front element, the back mirror reflects it forward to another mirror, which in turn reflects the light back through a hole in the back mirror to the film plane.

On some lenses the curve on the front element reflects light through still another optical system so the lens performs as if it were equipped with a teleconverter, doubling the focal length. This reduces the weight even further, enabling a 250mm lens to become a 500mm lens.

There may be a slight difference in efficiency between mirror and standard telephoto lenses. In a standard lens, light enters the front, passes through a series of lenses and is focused onto the film plane. The f-number is relatively close to the light transmission value of the lens. Thus, an f-8 lens has a speed at or very close to f-8.

In a mirror lens, however, the f-number is slightly misleading. Because of the "blind spot" in the center of these lenses (where the mirrors are), the light transmission value is a bit less than the marked aperture. An f-8 mirror lens may well be closer to f-9 or f-9.1.

This is not a problem in cameras with a TTL (through the lens) metering system. Since the light is read directly off the film plane, a programmed shutter can compensate correctly. Because of this, you'd have to do some comparison shooting to see any major difference.

With a mirror lens, you can't change the aperture except with the use of filters, such as neutral density. These, however, fit on the back of the lens, and are readily available and easy to use.

Two other things: With a mirror lens you have to take the shutter speed the camera system gives you. The way to change it is to use different speed films -- not a big problem since the introduction of the super-fast films. Also, due to the lens design, points of light that are out of focus on a picture often show up as "doughnuts."

I have been using a Ricoh 500mm mirror lens with great success. I use a tripod with it, but find it a wonderful addition to the camera bag.

Q. My husband has an Olympus OM-1 and I have an OM-G. The G is approximately six years old and had to be repaired last year as a result of being cleaned (the focusing prism was knocked out of kilter and the mirror was bent and sticking). Repairs totaled more than the purchase price.

How much more money do you think I should invest in repairs for this camera before giving up on it? And, when I do change, to what do I change? My husband and I share five lenses with OM mounts. We have had no problem with the OM-1.

A. I don't think I would put any more money into that G. My rule of thumb is: If it costs more than one-third to one-half of the price of a new one to repair it, no matter what it is, I get a new one.

You should look for another OM body; take a look at the OM-88, or keep your eyes open for a used OM-1.

Normally I don't recommend buying used equipment unless you know the past owner and how it was maintained. But I have seen some OM-1s in good shape for not a lot of money. Remember, if you don't know the past owner, be sure you buy from a dealer you trust.