Canon Speedlite 600EX II-RT Manual (en)

The Speedlite’s built-in external metering sensor measures the flash 
light reflected from the subject in real time and automatically stops the 
flash firing when the standard exposure is reached.
“Auto external flash metering” can be used with the EOS DIGITAL 
cameras released in and after 2007. “Manual external flash metering” 
can be used with all EOS cameras.
You can shoot in fully automatic flash mode. The flash output is 
automatically adjusted according to the ISO speed and aperture set on 
the camera.
Set the flash mode to </>.
Press the <E> button.
Turn <9> to select <r>, then 
press <8>.
When you press the camera’s shutter 
button halfway, the effective flash 
range is displayed.
//.: Flash External MeteringN
/: Auto External Flash Metering
Effective flash range
When </> is set, flash exposure compensation (p.28) and FEB 
shooting (p.29) can be performed.
If <r> is not displayed, set the flash Custom Function to C.Fn-05-2