Digital SLRs are becoming increasingly popular. With so many lens choices, it is important to know your options before purchasing lenses for your camera.

With a 35mm film camera, choosing a lens is fairly simple. A 35mm lens captures images as you see them. Shorter lenses take wide-angle pictures. For example, 28mm, 24mm and 20mm are common focal lengths for wide-angle lenses.

Lenses with focal lengths greater than 35mm are telephoto lenses. They allow you to take close-up photos without getting closer to your subject.

Unfortunately, choosing lenses for a digital SLR is more complicated. That's because the sensor that captures images is usually smaller than a frame of 35mm film. Lenses, on the other hand, are still labeled for 35mm film cameras. That is, unless you buy a digital-specific lens. I see few of these advertised.

Because digital sensors are smaller than film frames, images are magnified. This is called field of view crop.

Field of view crop is also called the focal length multiplier or multiplication factor. Common values for field of view crop range from 1.3 to 2. You multiply a lens' focal length by the multiplier to determine its 35mm equivalent.