Q - What are the pros and cons of a regular telephoto against a mirror type?

A - The practical difference is in the weight and length of the lens. The mirror, or catadioptic, lens is shorter and lighter than the equivalent telephoto because the image is internally reflected by mirrored surfaces. This makes it fine for hand holding for sports action or nature shooting.

The disadvantage is that the mirror lenses can't be stopped down or opened up. You have to shoot at the marked f-stop. A 500-mm f8 mirror lens is just that - period. This isn't bad when you're shooting action where you need only one plane in focus, but it can be limitation when you want overall sharpness, as in landscapes. Your choice depends on the kind of pictures you take. Neither is the complete answer.

Q- I recently bought a new Nikon EL-2 but I'm keeping my old lenses. Do you know where I could have them modified so that they fit my new camera? Also, if I have them changed could I still use them on the older-model bodies?

A - Nikon has designated modification centers where you can send your older lenses to be altered to fit the new models. The cost varies with the type of lens. Most will run about $18.50, but a zoom will cost $33.50 to modify and a fisheye even more. Not all lenses require the same adaptation. Some need as many as five ktis for this change. For this reason you should send the model and serial number of your lens to one of these modification centers to find out if your lens can be altered.

Modification Centers: Los Angeles, ACS, Nikon Repair, 13839 Ventura Blvd., Sherman Oaks, Ca. 91423; San Francisco, Nikon, 501 Folsom Street, San Francisco, Ca. 94105; Chicago, Nikon, 7550 North Oak Park Ave., Niles, Ill., 60648; New York, Nikon, 623 Stewart Ave., Garden City, N.Y. 11530.

You'll be able to use your modified lenses on both the old-and new-model Nikons.