Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 129)

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3 Customizing AF Functions
If focus cannot be achieved with autofocus, you can have the camera
keep searching for the precise 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 precise focus.
OFF: Stop focus search
If autofocus starts and the focus is far
off or if focus cannot be achieved, the
lens drive will not be performed. This
prevents the lens from becoming
grossly out of focus due to focus
search drive.
Lens drive when AF impossible
When using super telephoto lenses or other lenses with wide focusing drive
ranges during focus search, the focus may become blurred and it may take
more time to achieve focus next time. Setting [
Stop focus search ] is
recommended.