Q. I would like to know how to do print solarization and other darkroom techniques. Where can I learn?

A. Solarization is more correctly called the Sabattier effect. The technique is to re-expose a partially developed print and then continue to develop. The effect will be to start reversal of the positive to a negative image. The degree of reversal depends on the strength of the light used to flash the partially developed print and how far along the print was developed before flashing.

A way to start the solarization or Sabttier effect is to make a print through your enlarger from a negative that has strong sidelighting. Expose the print somewhat under (about 20 percent under) and then, when the print is about one-third into normal development, flash on a 10-watt bulb from a distance of about four feet from the developing tray. This exposure will start the reversal process. For the full effect continue to develop the print to normal time.

There are a number of inexpensive special-effects photo books in photo stores, like "Petersen's Special Effects" by Ken Biggs at $2.95 and H. P. Books' "How to Create Photographic Special Effects" by Allan Horvath at $7.95. Look them over for many ideas you may want to try.

Q. What's the best way to lighten underexposed slides?

A. The only way I know of is by copying, and overexposing the copy. This won't give you a perfectly exposed slide, but it will help correct the underexposure. You should try to overexpose by about the amount of underexposure on the original -- e.g., one stop -- by opening up from f/8 to f/5.6.

Recopying will take some experimentation, because there's a color shift and a loss of crispness in the process. The best material to use for this is Extachrome slidecopying film, made just for this purpose.

Q. What's the difference between using a soft-focus lens or filter over the lens and using a soft-focus filter over the enlarging lens when making a print?

A. A diffuser in front of the camera lens will cause the light to spread from the highlights into the shadows, while a diffuser in front of an enlarger lens does just the opposite -- that is, the darker (shadow) areas will spread into the lighter (highlight) areas.

There are many different kinds of diffusers. A soft-focus aperture disc is a metal filter with a large central hole surrounded by a pattern of smaller holes. This system gives a controllable diffusion as the disc is rotated. Another diffuser disc is of clear plastic with adjustable blades like a shutter.

The handiest diffusers for 35-mm shooters are the diffusion filters that fit right over the lens. The diffusion on these can be controlled by changing the lens' f/stop.

A cheap, and very effective, lens diffuser can be made from a nylon stocking; black is best to prevent the light from scattering. Simply stretch the nylon over a piece of cardboard with a round hole cut out to serve as the filter.

Q. What background, light and film make the most visible combination for typewritten lecture slides?

A. Any of the light primary colors are good background for typewritten titles. Yellow has the highest visibility. The typewritten title can be copied on any color slide film.

If you want an unusual and brillant effect, try copying the typewritten title, on white paper, with high-contrast litho film, which comes in 35-mm size. Use the resulting negative with a piece of colored gel background sandwiched to it, to show colored letters on a black background.