Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn I -14     Apply shooting/metering mode
While holding down the <A> (AE lock) button, you can switch to the 
registered setting (shooting mode, metering mode, shutter speed, aperture, 
or exposure compensation).
Disable: Pressing the <A> button will lock the exposure (AE lock).
Enable: By holding down the <A> button, you can instantly switch to the 
registered setting.
Register: Set the desired setting for the AE lock button: Shooting mode, 
metering mode, shutter speed, aperture, or exposure 
When you select [Register], [With AE lock button (AF on/AF 
] will be displayed where you can set the <A> AE lock button 
to also execute AF or not. Select [AF on/AF off] to register the 
setting to the camera.
C.Fn I -15    Flash sync. speed in Av mode
You can set the flash-sync speed for flash photography in the aperture-
priority AE (f) mode.
0: Auto
The flash sync speed is set automatically within a range of 1/300 sec. to 
30 sec. to suit the scene’s brightness. High-speed sync can also be 
1: 1/300-1/60 sec. auto
Prevents the sync speed from being set too slow under low-light 
conditions. It is effective for preventing subject blur and camera shake. 
However, while the subject will be properly exposed with the flash, the 
background may come out dark.
2: 1/300 sec. (fixed)
The flash-sync speed is fixed to 1/300 sec. This more effectively 
prevents subject blur and camera shake than with setting 1. However, 
the background may come out darker than with setting 1.
This setting cannot be registered if the shooting mode is set to “Bulb”.
When 1 or 2 is set, high-speed sync cannot be used with an external