Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 171)

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Flash Function Settings
Built-in Flash Function Settings
M
 Flash mode
Set to [E-TTL II] to shoot in E-TTL II/E-TTL
fully automatic fl ash mode.
Set to [ Manual fl ash] to specify your
preferred fl ash output manually.
 Shutter synchronization
Normally, set this to [1st curtain] so
that the fl ash fi res immediately after the
exposure starts.
Set to [ 2nd curtain] and use low shutter
speeds for natural-looking shots of subject
motion trails, such as car headlights.
o When using second-curtain synchronization, set the shutter speed to
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 [2nd curtain] is set.