Q. Some weeks ago I was at the airport to make pictures of a hot-air balloon race. Mostly I used my 50- mm lens. Those balloons were bigger than I thought. But I also wanted some close-ups, so I switched to a telephoto zoom. As I was changing lenses, however, I noticed that the mirror of my camera was fogged. I didn't have any lens tissue with me, so I couldn't wipe it dry. I tried blowing on it, but that didn't help. What was going on?

A. Nothing strange. The fog on your mirror had to come from a rapid change of temperature or humidity. If you were inside and discovered this condition just after coming outside, that's normal. It happens often, especially on cool, wet fall days, or cold winter days. It can also happen in summer if you go from a hot, humid outdoors to an air-conditioned indoors.

The fog also may have been caused by your blowing inside the camera. Warm, moist breath on a cool morning means fog.

The best thing about this experience was that you didn't have any lens tissue. Never, never, never shove anything into your camera, and especially don't touch that mirror. It scratches very easily.

Find a warm, dry room and let the mirror clear on its own. If you're carrying canned air in your bag (a good idea, by the way) you can clear your mirror with that. Just be careful not to use too much pressure. TELE-EXTENDERS

Q. I have a two-year-old Pentax that works swell, but I only have a 50-mm lens. I would like to have a telephoto lens but can't afford it right now. It has been suggested that I buy a tele- extender. I was told that this device could make my 50-mm lens work like a 100-mm lens. Is that true? Are these gadgets new? Are they expensive?

A. Most professionphotographers don't use tele-extenders very much anymore, but many still carry one in their camera bag.

A tele-extender is an optical device that looks somewhat like a lens, and fits between a camera body and its lens. It does have the effect of doubling the focal length of the lens. A 50-mm lens, used with a tele- extender, functions like a 100-mm lens. You are able to convert a light 200-mm lens into a far-reaching 400. That's great.

But there are some grave disadvantages.

First of all, a tele-extender will reduce the original lens' speed by two f stops. That is, if your lens is f 2.8, it becomes a 4.5 with a tele-extender. Much more of a problem is if your lens is f 4.5 to start. With a tele-extender it becomes f 6.3 -- too slow for many situations.

Further, with the loss of lens speed it becomes more difficult to focus.

Tele-extenders can serve as an acceptable alternative to some of the bigger, more expensive zoom lenses. Even in that case, however, you certainly will only get what you pay for. Stay with the ones made by the camera company whose product you're using, and don't buy the least expensive thing you can find. You can get a fairly good tele-extender for about $100. PHOTO SEMINAR

On Sunday, November 17, The White House News Photographers Association will again join forces with the Smithsonian Institution for the Ninth Annual Student Photo Seminar.

It will be held from noon till 5 in the Carmichael Auditorium of the Museum of American History, 13th Street at Constitution Avenue NW. The event is free and open to area high school students.

A panel of professionals from newspapers, magazines and television networks will discuss their specialties, and question-and-answer periods will follow. Students are encouraged to bring their own pictures for critiquing by these pros. For more information, call 683-2557.

Carl Kramer, former director of photography for The Washington Post, will try to answer your photography questions in his column, but cannot reply individually. Send your questions to: Carl Kramer, c/o Weekend, The Washington Post, 1150 15th Street NW, Washington DC 20071.