Canon EOS 5D Mark IV L-series Zoom Lens Kit Owner's Manual

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 Setting the Flash FunctionN
290
You can set the flash-sync speed for 
flash photography in the <f> aperture-
priority AE mode.
 
: Auto
The flash sync speed is set automatically within a range of 1/200 
sec. to 30 sec. to suit the scene’s brightness. High-speed sync is 
also possible.
 
: 1/200-1/60sec. auto
Prevents a slow shutter speed from being set in 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.
 
: 1/200 sec. (fixed)
The flash sync speed is fixed at 1/200 sec. This more effectively 
prevents subject blur and camera shake than with [1/200-1/60sec. 
auto
]. However, in low light, the subject’s background will come out 
darker than with [1/200-1/60sec. auto].
Flash Sync. Speed in Av Mode
If [1/200-1/60sec. auto] or [1/200 sec. (fixed)] is set, high-speed sync is not 
possible in the <f> mode.