Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 207)

Shutter synchronization
Normally, set this to [ First-curtain synchronization ] so that the flash fires immediately after
the shooting starts.
Set to [
Second-curtain synchronization ] and use low shutter speeds for natural-looking
shots of subject motion trails, such as car headlights.
Set to [
High-speed synchronization ] for flash photography at higher shutter speeds than
the maximum flash sync shutter speed. This is effective when shooting with an open
aperture in <
> mode to blur the background behind subjects outdoors in daylight, for
example.
Caution
When using second-curtain synchronization, set the shutter speed to 1/50 sec. or
slower. If the shutter speed is 1/60 sec. or faster, first-curtain synchronization will
be applied automatically even if [
Second-curtain synchronization ] is set.
Flash exposure compensation
Just as exposure compensation is adjustable, you can also adjust flash output.
Note
If flash exposure compensation is set with the Speedlite, you cannot set the flash
exposure compensation with the camera. If it is set with both the camera and
Speedlite, the Speedlite's setting overrides the camera's.
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