Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 791)

Shutter Button Function for Movies
You can set the functions performed by pressing the shutter button halfway or completely
during movie recording.
Caution
In movie recording, the [ Shutter btn function for movies ] setting overrides any
function assigned to the shutter button in [ 6 : Custom Controls ].
1.
Select [ : Shutter btn function for movies].
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