Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 337)

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3 Menu Function Settings
Silent control
When [ Enable h ] is set, you can use the < h > touch pad with the
Quick Control to change settings while suppressing the operation
sound during movie shooting. For details, see page 325.
V button function
You can set the functions performed by
pressing the shutter button halfway or
completely during movie shooting.
If [ / k ] or [ q / k ] is set, besides pressing the < 0 > button,
you can start/stop the movie shooting by pressing the shutter button
completely or by using Remote Switch RS-80N3 or Timer Remote
Controller TC-80N3 (both sold separately, p.258).
Setting Pressing halfway Pressing completely
/– Metering and AF No function
q /– Metering only No function
/ k Metering and AF
Starts/stops movie
shooting
q / k Metering only
Starts/stops movie
shooting
During movie shooting, the [ V btn function ] setting overrides any function
assigned to the shutter button with [ 8 6: Custom Controls ].