Canon MARK IV User Manual (en)

Custom Function Settings
C.Fn II -6     Shutter curtain sync.
0: 1st-curtain synchronization
1: 2nd-curtain synchronization
The flash fires immediately before the exposure ends. When this is 
combined with a slow sync speed, you can create a trail of light such as 
from car headlights at night.
This Custom Function can be used to obtain 2nd-curtain sync effects even with 
EX-series Speedlites which do not have this feature (except Speedlite 270EX).
C.Fn II -7     Flash firing
Enables or disables the firing of an external flash or non-Canon flash 
connected to the PC terminal.
0: Enable
1: Disable
Convenient when you want to use the external flash’s AF-assist beam 
only. Note that whether or not the AF-assist beam is emitted depends on 
the C.Fn III -15 setting.
C.Fn II -8     Viewfinder info. during exposure
0: Disable
1: Enable
The viewfinder information will be displayed even during an exposure. 
Displays the exposure setting, number of remaining shots, etc., during 
continuous shooting.
When using EX-series Speedlites having the shutter curtain synchronization 
function, C.Fn II -6’s setting does not take effect. Set the shutter curtain 
synchronization in the Flash functions settings menu (p.122) or directly on the flash.
When using a Speedlite 270EX, shutter curtain synchronization can be set in the 
Flash functions settings menu.
If 1 is set and you press the shutter button completely, note that the flash will fire a 
total of two times right before the exposure ends.
When using 2nd-curtain sync, set the shutter speed to 1/50 sec. or slower. If the 
shutter speed is 1/60 sec. or faster, 1st-curtain sync will be applied even if 1 is set.
When the shooting mode is set to “Bulb”, even if setting 1 is set, it will not 
take effect.