Canon Speedlite 600EX-RT - Manual (Page 27)

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With high-speed sync, 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.
Display <c>.
Press function button 4 <Y> to
display <c>.
Check that <F> is lit in the
viewfinder.
c High-speed Sync
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 4 <Y> to
turn off <c>.
High-speed sync is not available during stroboscopic flash.
When using the flash with EOS cameras compatible with E-TTL and
released up to 2011, high-speed sync is not possible with radio
transmission wireless flash shooting (p.51).
With high-speed sync, the faster the shutter speed, the shorter the
effective flash range will be. Check the effective flash range on the LCD
panel.
COP
Y