Canon EOS-1 DX Mark III Advanced User's Manual page 255

2.
Set the recording mode to <
Press the <
selection.
3.
Focus on the subject.
Before recording a movie, focus with AF (
By default, [
camera always keeps focusing (
When you press the shutter button halfway, the camera will focus with
the current AF method.
>.
> button, then turn the <
: Movie Servo AF] is set to [Enable] so that the
).
255
> or < > dial for
) or manual focus ( ).