The EOS 60D (my camera at the moment) offers a range of three Auto focus (AF) modes (One Shot, AI Focus and AI Servo).
One shot — for still subjects
AI Focus — automatic switching of AF mode
AI Servo — for moving subjects
This is a really good feature to know about and I suggest you learn it. It can make the difference between getting the shot and not getting the shot. Personally, I don’t see much of a reason to use AI Focus. Trusting the camera to do the right thing may be detrimental to your photography experience.
From the manual:
One-Shot AF for Still Subjects:
Suited for still subjects. When you press the shutter button halfway, the camera will focus only once.
- When focus is achieved, the AF point which achieved focus will flash briefly in red, and the focus confirmation light <o> in the viewfinder will also light.
- With evaluative metering, the exposure setting will be set at the same time focus is achieved.
- While you hold down the shutter button halfway, the focus will be locked. You can then recompose the shot if desired.
- If focus cannot be achieved, the focus confirmation light <o> in the viewfinder will blink. If this occurs, a picture cannot be taken if the shutter button is pressed completely. Recompose the picture and try to focus again.
AI Focus AF for Automatic Switching of AF Mode:
AI Focus AF switches the AF mode from One-Shot AF to AI Servo AF automatically if the still subject starts moving.
- After the subject is focused in the One-Shot AF mode, if the subject starts moving, the camera will detect the movement and change the AF mode automatically to AI Servo AF.
- When focus is achieved in the AI focus AF mode with the Servo mode active, the beeper will sound softly. However, the focus confirmation light <o> in the viewfinder will not light.
AI Servo AF for Moving Subjects:
This mode is for moving subjects when the focusing distance keeps changing. While you hold down the shutter button halfway, the subject will be focused continuously.
- The exposure is set at the moment the picture is taken.
- When the AF point selection is automatic, the camera first uses the center AF point to focus. During autofocusing, if the subject moves away from the center AF point, focus tracking continues as long as the subject is covered by another AF point.
- With AI Servo, the beeper will not sound even when focus is achieved. Also, the focus confirmation light <o> in the viewfinder will not light.