Canon Speedlite 600EX-RT Instruction Manual - Page 27

c High-speed Sync 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 . Press function button 4 to display . Check that is lit in the viewfinder. 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. If you set a shutter speed that is equal to or slower than the camera's maximum flash sync speed, will not be displayed in the viewfinder. To return to normal flash shooting, press function button 4 to turn off . High-speed sync is not available during stroboscopic flash. 25