Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 264)

3 Setting the Flash Function
264
You can set the flash-sync speed for
flash photography in the aperture-priority
AE <
f > mode.
: 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.
: 1/250-1/60sec. 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.
: 1/250 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 Sync. Speed in Av Mode
If [ 1/250-1/60sec. auto ] or [ 1/250 sec. (fixed) ] has been set, high-speed
sync is not possible in the < f > mode.