Canon EOS-1D X Mark III Af Setting Manualbook page 61

[Auto selection AF] or [
shooting subjects with unpredictable movement that are
difficult to capture with [AF point expansion] or [Zone AF]
(figure skating jump scenes for example). Another effective
use is when you want to take action images of moving
objects without changing the composition.
After the AF point locks on to the subject you want you can
keep continuous shooting without largely moving the
camera (lens) making it possible to keep tracking the runner
without changing the composition.
This technique is effective when you want to include both
the subject and the background in the composition.
+ Tracking AF] are effective when
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