Canon EOS 80D - Manual (Page 236)

3 Setting the FlashN
236
You can set the flash-sync speed for
flash photography in the aperture-priority
AE <f> mode.
 4 : Auto
The flash sync speed is set automatically within a range of 1/250
sec. to 30 sec. to suit the scene’s brightness. High-speed sync is
also possible.
 6 : 1/250-1/60 sec. 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.
 7 : 1/250 sec. (fixed)
The flash-sync speed is fixed at 1/250 sec. This more effectively
prevents subject blur and camera shake than with [1/250-1/60sec.
auto]. However, in low light, the subject’s background will come out
darker than with [1/250-1/60sec. auto].
Flash Synchronization Speed in Av Mode
If [1/250-1/60sec. auto] or [1/250 sec. (fixed)] is set, high-speed sync is not
possible in the <f> mode with the external Speedlite.