Q. I have read with interest your articles on airport X-ray machines and I want to expand the discussion.

If you are visiting in another country for a while, would it be reasonable to get your film processed, by mail or otherwise, before you leave that country?

This would bypass some X-rays and add a safety factor.

A. Yes, that's a fine idea, but make very sure that the processor can be trusted. Get a recommendation. You should also be sure that your processing can be handled on the schedule you need. It would be terrible if you had to fly away before you had your film back.

In most of the European countries and in Japan you'll have no problem, but use care and don't trust strangers.

Q. I have recently noticed that the compressed air that I use contains propellant that may damage the earth's ozone layer. After shopping around, I have yet to find compressed air that doesn't contain this propellant. Are you aware of any brands that are safe? If not, can you recommend an effective alternative?

A. I checked with the major manufacturers and they confirm that their products do contain fluorocarbons. Several retailers told me that the amounts had been reduced to insignificant levels.

I don't believe that any amount of fluorocarbon is insignificant, so I'm switching to carbon dioxide.

CO2 can be purchased in steel bottles, about twice the size of the propane bottles used for torches in a home workshop. You'll need to cap it with a regulator and hose, but these are readily available.

The other thing to try is a setup designed for airbrushing. One of the major art supply houses in the area can give you specific advice, depending on your needs.

Q. I use daylight slide film ISO 125, 200 or 400 almost exclusively. Although I have seen tables telling me which blue filters should be used for certain kinds of indoor lighting, I still don't understand how to tell what type of lighting is 3200 Kelvin so that I can use an 80B filter, or which is 3400 K requiring an 80A filter.

For that matter, why do certain TV floodlights permit normal photography indoors without filters? Why don't I need a filter for flash?

A. In the absence of a $6,000 color temperature meter, I suggest you get a copy of Kodak's Color Data Guide available at most camera stores.

This book gives a temperature scale for many situations and helps simplify this complicated subject.

As to TV lights, generally they are halogen or quartz lights that are balanced for 6,000 Kelvin, or high noon. This alleviates the need for filtration in most films. Flash has this same noon balance.

Q. I have been doing 35mm photography since 1971 and have narrowed my interest to glamor, fashion and some nature photography.

I have been considering the purchase of a medium-format camera.

I have had some experience with large formats, and don't want to go back to that kind of equipment.

I have been told that there is a medium-format camera that changes backs to acceptvarious film sizes. Do you know which it is?

Do you have a preference in the medium range that would best suit the type of shooting I do? What about costs? Should I try to start off with a used camera?

A. You will find the medium format very exciting. The main advantage is in quality. You have an image roughly double the size of 35mm, and the quality is improved accordingly.

Several of these cameras offer exceptional versatility, with interchangeable backs of varying sizes, such as 6x6 cm (the biggest), 6x4 1/2, and even 35mm. One of the best of these is the Bronica SQA. Equipped with a Speed Grip for rapid advance and a normal lens (80mm, f2) this will make a fine outfit for fashion shooting. It will cost about $1,600, but you're talking long-term investment.

When looking, be sure you check out the Hasselblad. This is one of the world's premier cameras. You'll probably pay about twice as much for it but you're buying what is considered the best in the field.

A reputable camera store may have some used equipment, but it could cost two-thirds as much as new gear.

Q. I have a Minolta 7000 AF. I was planning to buy a 2X teleconverter and use it in the manual mode. A neighbor tells me that I will have to purchase an adapter as well as the Teleconverter. He also doubted that I could use my Minolta 70-210mm zoom with this setup. Will this work?

A. The state of the art has caught up with you. There has just come on the market a new adapter that handles this problem. There are two, a 1.4X and a 2X. They are made by Rokunar, and sell for about $80. These adapters enable you to use your Minolta lenses and gain that focal length extension. They are available in camera stores.

Here are some hints from Nancy Salvio of Greensburg, Pennsylvania:

When photographing flowers, say when I'm visiting public gardens, I carry a misting bottle to create instant dew. It's easily refillable in restrooms, etc. My husband tells everyone it's to cool him off.

An emergency or make-do soft focus can be created with a tissue (Kleenex) with a hole poked in the center. It's worked well for me and my 35mm camera.

When posing someone for a portrait, I have them first look away and then turn toward the camera with a fresh smile. I have them do this on my cue instead of sitting, smiling and posing artificially. This allows the subject to relax and makes for a much better portrait.