Canon MARK IV User Manual (en)

 Setting the Flash
Functions settable under [Flash function settings]
On the screen, the settable functions and what’s displayed will 
differ depending on the Speedlite, current flash mode, flash 
Custom Function settings, etc.
To see which functions your Speedlite provides, refer to the Speedlite’s 
instruction manual.
Flash mode
With an external Speedlite, you can select the flash mode to suit 
your flash shooting.
[E-TTL II] is the standard mode of 
EX-series Speedlites for automatic 
flash shooting.
[Manual flash] is for users who want 
to set the [Flash output] (1/1 to 1/
128) themselves.
[MULTI flash] is for users who want 
to set the [Flash output], 
[Frequency], and [Flash count
For [TTL], [AutoExtFlash], and 
[Man.ExtFlash], refer to the instruction 
manual of the Speedlites which 
provide the respective flash mode.
Shutter sync.
Normally, set this to [1st curtain] so that the flash fires immediately 
after the exposure starts.
If [2nd curtain] is set, the flash will fire right before the shutter closes. 
When this is combined with a slow sync speed, you can create a trail 
of light such as from car headlights at night. With 2nd curtain sync, 
two flashes will be fired: Once when you press the shutter button 
completely, and once immediately before the exposure ends.
If [Hi-speed] is set, the flash can be used with all shutter speeds. 
This is especially effective for portraits using fill flash when you want 
to give priority to the aperture setting.