Q - Sometimes I see a copyright sign with the photographer's name under a picture. What does this mean?

A - This indicates - or should indicate - that the photographer has taken out a copyright on his material to prevent others from using it without permission or without paying him.

To copyright a picture (or a whole contact sheet), send a copy of the work with a completed and signed Application Form J and a $6 fee to Register of Copyrights, Library of Congress, Washington, D.C. 20540. Copies of the form are available free from the Library of Congress, and from certain office supply stores catering to the legal profession. A new copyright law - the first since 1909 - went into effect this year. For full details on the law, check with your local library for a copy.

Q - I can't seem to get consistent exposures with my electronic flash. Sometimes they're too dark and at other times too light. I use the recommended guide number of 30 with ASA 25 film. What's wrong? Also, how can I determine the guide numbers for films with other ASAs, such as ASA 64, 80 and 100?

A - Shooting flash pictures is an exact science only in the laboratories where the tests are made to determine the recommended guide number. Since we don't take our photos in a lab under controlled conditions, our results will vary.

Moving indoors or out, the size of the room, the color of the walls - all effect the exposure. There can be as much as a full stop difference between flashing in a small white room and a large gym.

Add to these your own personal preference as to what you consider a good exposure as against another opinion, and the tendency of some manufacturers to "upgrade" their guide number.

Because of all this, most serious photographers make their own tests with their own equipment under the shooting conditions they are going to use.

It's quite simple to figure out your own guide number. Simulate your own lab setup by shooting a series of flash exposures in the kind of room you usually take pictures in. Use a model so you can judge skin tones.

Place your model a measured 10 feet from your flash and make a series of flash exposures. Start with the recommended guide number. If it's 30, set your f-stop to 2.8 and flash, then make four more exposures with a stop differencea at f2, f1.8, f4 and f5.6. Marl each f-stop on a piece of paper and have the model hold it in front so you can identify each exposure later.

If you also plan to take outdoor flash, repeat the same procedure outside.

After you have these tests developed, compare the results on a light table and choose the one you like. Then all you have to do is multiply the f-numver by 10 and - Viola - instant guide number. If your judgement is a between f-stops, guestimate. Remember, it's not a science but an art.

When you have arrived at your guide number this way, don't let anyone else confuse you with theirs - stick to yours.

Re-figuring the guide number to a new ASA is just as easy as arriving at your own. Keep in mind the same 10-foot principle that you used for the flash test.

Each time you double the ASA of the film, stop down one stop and multiply by 10 on the 10-foot distance setting. That is: If the guide number is 30 at ASA 25, at 10 feet the setting would be approximetly f2.8. If you change film to ASA 50, then at the 10-foot distance you would stop down to f4, which would than make your guide number 40 for the ASA 50 film. This is a rule-of-thumb system.

If you want to be dead on, and have a pocket calculator with a square-root function, you can come closer. The formula is: Divide the new ASA by the old, take the square root of the result and multiply it by the old guide number for the new one. As an example: If your old ASA was 25 with a flash guide number of 30, and you want to change to an ASA 64 film, then 64/25=2.56, the square root of which is 1.6; multiplied by the old guide number of 30, this gives you a guide number of 48 for the ASA 64 film. Then at 10 feet your setting would be a little past f4.

The rule-of-thumb system would give you about the same answer. That is: With a guide number of 30 you would shoot the ASA 25 film at f2.8, which then you would stop down 1 1/2 stops to between f4 and f5.6, and not wear out your pocket calculator.