Canon EOS-1DX Af Setting Manualbook page 59

Using this setting shifts the focus point used from a
manually selected AF point, to an adjacent (up, down,
left , and right, or surrounding) AF point, to aid focus
tracking. When using this setting it is easier to obtain the
desired composition as the subject is captured centering
around the manually selected AF point. Based on the
subject's movement characteristics, (i.e. likelihood of
subject moving from the selected AF point) and the size
of the subject within the frame, select either [AF point
expansion] or [AF area selection mode]. In addition,
when the subject has a lot of movement, setting Case 5
or Case 6 from the AF Configuration Tool is also
recommended.
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