Q. I would like to learn to take soft-focus portraits. Can I do this with a normal lens? I really can't afford a heavy investment, although I can spend $25 or preferably less.

I'd like to shoot these in both color and black-and-white. Since I am now doing my own black-and-white darkroom work, can I gain the effect through processing or printing?

Also, please recommend techniques for indoor and outdoor lighting.

A. Soft-focus portraits can be lots of fun, and, yes, you can do them for much less than $25.

One of the systems used for years is the "petroleum-jelly-on-the-skylight-filter" method.

Here's what to do: Remove the skylight filter from your lens. Then, with a cotton swab, apply a small amount of the petroleum jelly to the outer edges of the glass on the front side of the filter. Use your finger to spread the jelly, keeping a heavier layer near the outside and a thinner layer at the center. Put the filter back on the camera. Then clean your hands thoroughly.

Be extremely careful that you get no jelly on your lens. It could be a mess to clean off. (To clean your filter, take it off your lens, wipe it with an alcohol-soaked cotton ball, then polish it with a lens tissue.)

If you are shooting black-and-white, stay with a contrasty film such as Panatomic X or Plus X. If you're using color, stay with the ISO 100 film. Overexposing by half a stop can help.

In the darkroom, when dealing with black-and-white negatives taken this way, be prepared to use a slightly harder paper. Focus your enlarger carefully; I suggest a grain-focusing viewer.

Since you are blessed with your own printing facilities, you can shoot your pictures without the petroleum jelly. Then, while you are exposing a print, "dodge" the entire image with some Cellophane or clear plastic. A cigarette pack cover works. Crumple it up very tightly, flatten it out some (not much) and go to work. Let the light from your enlarger pass through the plastic onto your paper -- the results will delight and amaze you.

As to lighting, try to shoot with a somewhat contrasty light. You need separation in your subject. The softness can come from the methods described. FIGHTING MOISTURE

Q. I'll be traveling in Southeast Asia for most of June. Do you have any tips on protecting my camera and film from the heat and dampness of the tropical climate? Also I read recently that exposed film that passes through airport X-ray machines can become fogged and ruin the photographs. Is this true?

A. A couple of suggestions for this kind of travel, not only in Southeast Asia, but also in Florida, New Orleans and along the Gulf Coast:

First buy a container of moisture-absorbing material such as Silica-gel and keep it in your camera bag. Most good camera stores carry it. A fairly large container costs about $7.As it absorbs moisture, it turns pink. Place it in the hot sun (or an oven) and it will dry out and turn blue again.

To further protect your film from heat and moisture, you can buy an insulated bag such as is used to carry baby bottles. They're not heavy or expensive.

As for airport X-rays: Yes, they can damage film, especially with repeated exposure. A good way to beat this is to buy a re-usable, lead-lined bag and use it at airports. It's light and only costs about $11.

Have a good trip. NATURE WORKSHOP

On Wednesday, a free nature photography workshop will be presented under the auspices of the Arlington County Nature Centers. Slide show producer and photography teacher Joshua Taylor Jr. will offer a comprehensive, two-hour program. It will cover the use and selection of lenses, composition, lighting with strobe and reflectors, filter use and close-up shots.

The program will be held from 7:30 to 9:30 p.m. at the Gulf Branch Nature Center, 3608 N. Military Road, Arlington. For reservations, call 558-2340.