Canon EOS-1D X Mark III Advanced User's Manual page 788

2.
Select an item.
Half-press
Specify the function performed by pressing the shutter button halfway.
Fully-press
Specify the function performed by pressing the shutter button
completely.
With [Fully-press] set to [Start/stop mov rec], you can start/stop movie
recording not only with the movie shooting button but also by pressing the
shutter button completely, or by using Remote Switch RS-80N3 or Timer
Remote Controller TC-80N3 (both sold separately).
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