Canon EOS 5D Mark III Instruction Manual (en)

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: Selecting the AF ModeN
This AF mode is suited 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 area selection mode is set to 61-point automatic selection (p.72), 
the camera first uses the manually-selected AF point to focus. During 
autofocusing, if the subject moves away from the manually-selected AF point, 
focus tracking continues as long as the subject is covered by the Area AF frame.
AI Focus AF switches the AF mode from One-Shot AF to AI Servo 
AF automatically if a still subject starts moving.
 
After the subject is focused in 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.
AI Servo AF for Moving Subjects
AI Focus AF for Automatic Switching of AF Mode
 
If focus cannot be achieved, the focus confirmation light <o> and AF 
status indicator <
e> in the viewfinder will blink. If this occurs, the 
picture cannot be taken even if the shutter button is pressed completely. 
Recompose the picture and try to focus again. Or see “When Autofocus 
Fails” (p.112).
 
If [z1: Beep] is set to [Disable], the beeper will not sound when focus 
is achieved.
 
After achieving focus with One-Shot AF, you can lock the focus on a subject 
and recompose the shot. This is called “focus lock”. This is convenient when 
you want to focus a subject not covered by the Area AF frame.
With AI Servo AF, the beeper will not sound even when focus is achieved. 
Also, the focus confirmation light <o> in the viewfinder will not light up.
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 up. Note that focus will not be locked in 
this case.