Canon EOS 5D Mark III Instruction Manual (en)

With the [22] to [25] menu tabs, you 
can set the AF functions to suit your 
shooting style or subject.
You can set the AF operation characteristics and shutter-release timing 
for the first shot during continuous shooting with AI Servo AF.
: Equal priority
Equal priority is given to focusing and 
shutter release.
: Release priority
Pressing the shutter button takes the 
picture immediately even if focus has not 
been achieved. This gives priority to 
getting the shot rather than achieving 
correct focus.
: Focus 
Pressing the shutter button does not take the picture until focus has 
been achieved. Effective when you want to achieve focus before 
capturing the shot.
 Customizing AF FunctionsN
22: AI Servo
AI Servo 1st image priority