Canon EOS-1D X Instruction Manual page 98

3 Customizing AF Functions
Lens drive when AF impossible
If focus cannot be achieved with autofocus, you can have the camera
keep searching for the correct focus or have it stop searching.
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.
98
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.