Canon EOS-1D X Mark III Af Setting Manualbook page 53

The switching of the AF point is based around the
selected AF point with [AF point expansion], this
makes it easy to concentrate on the desired
composition. The above image of the swimmer
coming up for air was captured with [AF point
Hints & Tips
[ Up, down, left, and right] and
[Around] according to the difficulty
of reading the movement, and the
importance to the central AF point.
When shooting subjects which are difficult to determine
the movement, the [surrounding] mode should be
selected, and when you want to capture the central
(manually selected) AF point as much as possible,
selecting the [ Up, down, left, and right] mode is
recommended.
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