A RECENT visit to the zoo presented me with a photographic challenge. I wanted my penguin pictures to look as though they had been taken in the wild. However, it was obvious to the trained eye that the birds' habitat -- a rocky cliff -- had been man-made. Also, I wanted to get striking portraits of these interesting creatures, portraits with a 3-D effect.

To meet the challenge and accomplish my goal, I used a photo technique called "selective focus," by which you can produce a photo with a sharp subject, and a blurred background or foreground.

The accompanying penguin photo was made using this technique. Knowing that telephoto lenses and low f-stops produce shallow depth of field, I decided to shoot with a 300mm lens set at f-5.6 on my 35mm SLR camera. Being able to look through the lens -- one of the main advantages of shooting with an SLR camera -- enabled me to quickly see that I had made the correct shooting choice.

When you are in similar shooting situations, selective focus can make the difference between a dramatic photograph and a snapshot. Remember, the background or foreground is just as important as the main subject, and you must be aware of how elements in these areas affect your picture.

As I mentioned, shooting with a telephoto lens set at a low f-stop can also blur the foreground. Therefore, if you are shooting through the bars or wire of a cage at the zoo, selective focus can blur these distracting foreground elements. In fact, it may blur them to the extent that they disappear from your photograph.

Selective focus is also beneficial in macro photography of flowers and plants, where it's often difficult to compose a scene with a non-distracting background. Here, too, it's advisable to select a low f-stop for shallow depth of field.

When shooting with telephoto or macro lenses, it's important to remember that camera-to-subject distance affects depth of field. When you've selected a low f-stop, the closer you get to a subject, the less depth of field you have. If you want to increase your depth of field, select a higher f-stop or move farther away from your subject.

Some 35mm SLRs have depth of field preview buttons that take the guesswork out of determining what will be in focus and what will be out of focus in your photo. By depressing the preview button before you take a photograph, you'll be able to see precisely how much of the scene will be in focus. That way, you'll avoid surprises when your pictures are developed and printed.

For beginning photographers, f-stops, depth of field and lens selection can often be confusing. If you are interested in learning more about the techniques of picture taking, look through Outdoor Photography magazine, which is available at newsstands. It offers clear and concise information, not only on the technical side of photography, but on how to apply technical knowledge in the field.

Rick Sammon writes for Associated Press Newsfeatures.