Canon EOS-1D X Mark III Af Setting Manualbook page 103

Another function that is effective for quickly switching AF
points while shooting is AF point registration and recall
feature possible using [Custom Controls]. There are several
methods to achieve this; one is to assign registered AF
points to a button via [Custom Controls] function. The
second method is AF point or AF area selection mode
registration. By carrying out either of the two options, you
can press a button and instantly switch to registered AF
points.
Registering frequently used AF points, or a strategically
placed AF point, enables instant response without the need
to reframe or alter the camera's position.
Further refinement in operation is possible, with the
button, and the
registered AF point] setting). These buttons provide the
possibility to [Switch only while pressed] or [Maintain
switching until pressed again] settings, making detailed
customization possible. When you want to return it to the
center after moving it a lot with
also effective without moving your finger. Using these
settings enables the camera to be tailored to suit the way
you shoot.
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button when set to ([Switch to
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Smart controller
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, it is