Canon EOS-1D X Mark III Advanced User's Manual page 656

6.
Focus on the subject.
When using autofocus
To start AF with [AF button] selected in the [AF for still photo
shooting] menu, select the (13) AF button.
To start AF with [Shutter button for AF/shooting] selected in the [AF
for still photo shooting] menu, select and release (4) Shutter button.
When focusing manually
To show/hide (25) MF button display, select (5) MF toggle button.
Select (25) MF button, then focus manually. To adjust focus toward the
foreground, use [
use [
Three levels of focus adjustment are available.
[ ] [ ]: Coarse
[ ] [ ]: Intermediate
[ ] [ ]: Fine
], and to adjust focus toward the background,
].
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