If you're a semi-pro in photography or would like to recover some of your photographic investment, how about selling stock photos?

An expert in this area is photographer Rohn Engh, who has successfully sold from his files for many years. Rohn publishes a newsletter telling what editors are buying and also conducts a nationwide photo workshop at which he personally explains the tricks of the stock picture trade.

For a sample of his Photoletter and a schedule of the workshops, write to: Photoletter, Osceola, Wisconsin 54020. FILTER FUN -- Amazing pictures can be taken with filters. Minolta has newly entered the filter field with the Cokin Creative Filter System. The system, developed by French photographer Jean Coquin, has some new ideas and applications for filter fans.

A filter holder (cost $2.99) is the heart of the outfit. It holds two filters in slots so that they can be combined. The holder will fit most popular 35-mm camera lenses of 49-mm, 52-mm or 55-mm size.

An excellent 40-page booklet in color that shows and explains the use of the filters available is included. It's a bargain, and even if you're only interested in a few filters the booklet will open your eyes to the pictorial possibilities with color, close-up, split-field, star filters and masks. Q: Just what does the term a "stop" mean? I've heard it used a number of ways. One is the range between ASA 64 and ASA 125 as a one-stop difference. Another is when you are told to decrease the development of film by 40 percent for each stop of overexposure. I've also heard that one can "push" film. How does this work? A: The term "f/stop" refers to the lens opening. The "f" numbers are determined by dividing the focal length of the lens by the diameter of the lens opening. The f/stops are marked to indicate the sizes of the lens openings as the aperture is closed down. (Each time that you close down the lens, such as from f/2 to f/2.8 -- or from f/4 to f/5.6, you get half as much light on the film.)

This same difference is kept between the ASA numbers, that is, from ASA 64 to ASA 125 the increase in light sensitivity is doubled -- or, there is the equivalent of one stop's difference -- the same as opening up the lens from f/5.6 to f/4.

When pushing film (setting the sensitivity rating higher than the recommended ASA) the same procedure is followed. If you want to double the speed of the film, such as from ASA 400 to ASA 800, you compensate by overdevelopment. And if you shot the film at a slower speed than recommended you have to develop the film a shorter time to adjust for the relative overexposure.

All you really have to keep in mind about f/stops, ASAs and shutter speeds is that they're related. A one-stop exposure difference can be made by doubling or cutting in half the f/stop, the ASA setting or the shutter speed. Q: I'm going on a five-day raft trip down the Colorado River. I have a Canon AE-1 and want to know if you feel that it's safe to take it with me. How can I keep the camera dry? If you don't advise me to take it -- then where can I get a rental, or insurance? A: You certainly will want to take your camera along as the scenery is fabulous, but you will have problems in keeping gear dry. The best type of camera for this use is a Nikonos, an easy-to-operate underwater camera. If you do decide to use one be sure to get the 35-mm lens that can be used both above and below water.

If you can't rent a Nikonos, the next best bet is to use a Haliburton case to carry your gear in. These are aluminum and come in assorted sizes. Their main feature is an O-ring rubber gasket that makes them watertight.

The final suggestion (and this won't cost a thing) is to wrap your camera and photo gear in a plastic bag -- the trip is occasionally through white water where you will have to protect your gear. The rest of the time you will want to use your camera during the run and especially when you stop for meals and an overnight stay. And finally, be sure to take the lead boat so that you can then shoot pictures of the others as they come through the rough stuff.

A reader who has taken several such trips had these comments to add:

In every instance the rafting company or trip concessionaire supplied all participants with WW-II ammunition boxes for cameras. These are very sturdy, roomy and have watertight gaskets. They work perfectly.

Halliburton cases are fine and will work, but are somewhat expensive unless one is planning to use them frequently.

Ammo boxes, by the way, can still be bought in surplus stores and would be an alternative way to protect camera gear -- on any outdoor trip.