For you Nikon users out there, here is a neat way to use the autofocus feature. This comes in handy for capturing fast sports/wildlife action. These are the settings you'll want to use:
Custom (pencil) menu:
Autofocus set to AF-S
AF area mode set to single
AE-L/AF-L set to AF ON
Compose your shot and set the focus by aiming the center focus icon at a definite target at the precise distance you want -- for example, a tree branch where a bird is about to land, or on second base where the baseball player's foot will land. Press the "AE-L/AF-L" button near the viewfinder. This will focus the lens, then let go of AE-L/AF-L button. Turn away,
press and hold the shutter button all the way down. Point the camera at pre-focused point when the subject approaches, and your camera will take the shot.
This method works much faster than human reflexes!