Canon Speedlite 600EX II-RT Manual (en)

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102
5
: Wireless Multiple-Flash Photography with Flash Ratio
3
Set the A:B flash ratio.
 
Press function button 3 <F>.
 
Turn <9> to set the flash ratio, then 
press <8>.
4
Take the picture.
X
The slave unit flashes at the set flash 
ratio.