Canon Speedlite 600EX II-RT Manual (en)

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With high-speed sync, you can shoot with a flash even at shutter 
speeds that exceed the maximum flash sync shutter speed. This is 
effective when you want to shoot in the aperture-priority AE <f> 
mode (open aperture) with background blur in locations such as 
outdoors in daylight.
Display <c>.
 
Press function button 4 <Y> to 
display <c>.
 
Check that <F> is lit in the 
viewfinder, then shoot.
c High-speed SyncN
With high-speed sync, the faster the shutter speed, the lower the guide 
number becomes. You can check the effective flash range on the LCD 
panel.
 
When the shutter speed is less than or equal to the maximum flash sync 
shutter speed, <F> is not displayed in the viewfinder.
 
To return flash firing to normal, press function button 4 <Y> to turn 
off <c>.