Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 203)

Flash Sync Speed in Av Mode
You can set the flash-sync speed for flash photography in < > mode.
: Auto
The shutter speed is set automatically within a range of 1/250 sec. to 30 sec. to suit the
brightness. High-speed sync is also possible.
: 1/250-1/60sec. auto
Prevents a slow shutter speed from being set automatically in low-light conditions. It is
effective for preventing subject blur and camera shake. Light from the flash provides
standard exposure for subjects, but note that backgrounds may be dark.
: 1/250 sec. (fixed)
The shutter speed is fixed at 1/250 sec., which is more effective in preventing 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 ].
Caution
High-speed sync is not available in < > mode when set to [ 1/250-1/60sec.
auto ] or [ 1/250 sec. (fixed) ].
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