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.
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.
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