Q. On a recent vacation I used a combination of a zoom (50- mm/macro) lens and the ISO 1000- speed film.

The 8x12 enlargements of the macro shots looked fine, but an 8x12 enlargement of a 250-mm shot taken in low light was much too grainy.

Under what conditions of light, focal length, etc., can the 1000 speed (or the 400 speed, for that matter) be used for acceptable enlargements?

Q. I read in a magazine that Kodacolor VR 1000 film gives good results at ISO 1000, but is far superior when overexposed by one stop. They said the same thing was true with Fujicolor HR 1600. You get good grain at ISO 1600, but results at 800 are sensational.

If this is the case, why don't they just label the film ISO 500 and ISO 800? Wouldn't that be more honest and helpful to a non-professional?

A. These two letters represent part of the continuing controversy on the value of high-speed color films

Opinions range from one purist referring to high-speed film as "trash," to the person who liked the film so much she was trying to use it instead of flash. She was shooting natural light with high-speed film at speeds sometimes as slow as ! or 1/4 of a second.

The fact of the matter is that high-speed color film is a tool, to be used as you would any other.

Is it grainy? Sure! But when the choice is to have a grainy picture or no picture, go for the grain.

Example: I like to shoot circus pictures. For a long time, I shot black- and-white and found that Tri-X, 400 ASA film was my best bet. I shot some Kodacolor and some Ektachrome, with limited success. When the Kodacolor 400 came out it was a step forward; it had speed, was pushable and had what I felt was acceptable grain on 8x10 blow-ups.

Then the high-speed films came along and we entered another phase. Now I could make high-wire and trapeeze pictures at speeds fast enough to stop the action. I could get shots with shadow detail good enough to see the elephants' faces during the finale parade.

Grainy? Sure! But I had pictures I hadn't been able to get before.

Use these films as needed. Be assured that they are being improved all the time. During the summer, the VR 1000 was improved. I made pictures with it at weddings and was delighted with the results. The uncropped 8x12s were fine.

I did notice, however, that when I had the chance to bracket my exposure (make one exposure by the meter, one exposure overexposed by one f stop and one exposure underexposed by one f stop) the overexposed shots frequently seemed best. In well over half the pictures, blow-ups were made from the overexposed frames.

I won't tell you that you should always shoot the high-speed films at a stop over. I will recommend that you make a couple of test overexposures on your next roll.

As to grain and telephoto lenses: It sounds as if your macro shots were with the lens set at 50-mm, probably in high-contrast light. On the other hand, it sounds as though your grainy, 250-mm shots were taken in low light. Low light can indeed make for a more grainy picture. And, with that 250-mm lens you are magnifying the grain 21/2 times.

Keep testing and you'll be able to decide on the conditions and exposures that best suit you.