Lens Tips

4. Consider "Auto AF"
Some cameras offer a third AF mode, which goes by such names as AI Focus AF and Automatic AF. In this mode, the camera employs single-shot AF unless the subject starts to move, at which point it automatically switches to continuous AF. This obviously can be very handy when photographing subjects that are likely to start moving. But it can also create a problem: If you move the camera while composing the scene, the camera will think the subject is moving, and switch to continuous AF. And when you use continuous AF with a stationary subject while hand-holding the camera, the AF system will keep trying to refocus if you jiggle the camera. It can be very frustrating to have focus locked in, and just as you start to shoot, you jiggle the camera slightly, so the AF system starts to refocus, giving you an out-of-focus shot. I use my cameras' AI Focus AF mode when I anticipate that a bird is about to take off, but not when I think the bird will stay put for a while (I don't always guess right, though). If I think it'll stay put, I use single-shot AF, or focus manually. For in-flight shots, of course, I use continuous AF, with just the center AF area active--see Tip 2.

Auto AF mode is ideal for stationary subjects that might suddenly start to move. This cormorant spent a while preening on the buoy, then suddenly decided to split. Photo by Mike Stensvold

5. Focus Manually
Don't forget that all the AF SLRs also allow you to focus manually, just as you would with a manual-focus SLR. If your AF system has trouble focusing on a particular subject--or you want to control precisely where the camera focuses--just switch to manual-focus mode, and do it yourself. AF SLRs give you the best of both worlds--the speed and convenience of autofocusing, and the ability to focus manually whenever you want to do so.

Some AF systems will focus on these plant wisps, some won't. This one wouldn't, so I did it manually. Photo by Mike Stensvold

With most AF SLRs, you have to switch to manual-focus mode to focus manually; trying to focus the lens manually while in AF mode can damage the focusing motor or linkage. But some lenses (Canon's USM lenses and Nikon's AF-S lenses, for example) allow you to adjust focus manually while in AF mode. This is a very handy feature. Check the instructions for your camera body and lenses to see if they allow manual focusing while in AF mode.

AF SLRs don't have central split-image and microprism focusing aids like those found in manual-focus SLRs, but it's still still pretty easy to focus them in good light using the ground-glass focusing screen. And the viewfinder's in-focus indicator will glow when focus has been achieved, just as it does in AF mode (assuming the light level and lens speed are adequate).

Next Month: Aerial Photography