Canon EOS 90D Advanced User's Manual page 176

Flash Function Settings
 Shutter synchronization
Normally, set this to [First-curtain synchronization] so that the fl ash fi res
immediately after the exposure 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 fl ash photography at higher
shutter speeds than the maximum fl ash sync shutter speed. This is eff ective
when shooting with an open aperture in < f > mode to blur the background
behind subjects outdoors in daylight, for example.
When using second-curtain synchronization, set the shutter speed to
o
1/80 sec. or slower. If the shutter speed is faster than 1/80 sec., fi rst-curtain
synchronization will be applied automatically even if [Second-curtain
synchronization] is set.
Flash exposure compensation

If fl ash exposure compensation is set with the Speedlite, you cannot set the
o
fl ash exposure compensation with the camera. Note that the Speedlite's
setting overrides the camera's if both are set at the same time.
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Just as exposure compensation is
adjustable, you can also adjust fl ash output
of external Speedlites.