Canon EOS 5D Mark III Instruction Manual (en)

 Customizing AF FunctionsN
If focus cannot be achieved with autofocus, you can have the camera 
keep searching for the correct focus or have it stop searching.
ON: Continue focus search
If focus cannot be achieved with 
autofocus, the lens is driven to search for 
the correct focus.
OFF: Stop focus search
If autofocus starts and the focus is far off 
or if focus cannot be achieved, the lens 
drive stops. This prevents the lens from 
becoming grossly out of focus due to 
focus searching.
You can change the number of manually selectable AF points. In the 
case of automatic AF point selection, all 61 AF points will remain active 
regardless of this setting.
: 61 points
All 61 AF points will be manually 
: Only cross-type AF points
Only cross-type AF points will be 
manually selectable. The number of 
selectable cross-type AF points will differ 
depending on the lens.
Lens drive when AF impossible
Selectable AF point
Super telephoto lenses can become grossly out of focus during continuous 
focus search, taking more time to achieve focus the next time. Therefore, 
setting [Stop focus search] is recommended for super telephoto lenses.