Canon ST-E3-RT-V2 Manual page 29

c High-speed Sync
With the high-speed sync function, the flash can synchronize with all
shutter speeds. This is convenient when you want to use aperture-
priority AE for fill-flash portraits of a subject.
When using the transmitter with EOS cameras compatible with E-TTL and

released up to 2011 or EOS REBEL T100/4000D/3000D, EOS REBEL T7/
1500D/2000D, EOS REBEL T6/1300D and EOS REBEL T5/1200D, high-
speed sync is not possible with radio transmission wireless flash shooting
(p.19).
With high-speed sync, the faster the shutter speed, the shorter the

effective flash range will be.
If you set a shutter speed that is equal to or slower than the camera's

maximum flash sync speed, <F> will not be displayed in the viewfinder.
To return to normal flash shooting, press function button 2 <Y> to

turn off <c>.
High-speed sync is not available during stroboscopic flash.

Using Fully Automatic Wireless Flash
1
Display <P>.
Press function button 4 to display

<P>.
2
Set the sync setting to <c>.
Press function button <Y> to set

<c>.
Check that <F> is lit in the

viewfinder.
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