Canon Speedlite 600EX II-RT Manual (en)

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91
: Optical Transmission Wireless Flash Photography
You can divide the slave units into two or three groups and perform 
E-TTL II/E-TTL autoflash photography while changing the flash ratio 
(flash output rate).
 
Autoflash Photography with Slave Groups
Wireless Multiple-Flash Photography
B
A
B
A
C
2 (A, B) groups
 (p.101)
3 (A, B, C) groups
 
To avoid interfering with transmission, do not place any obstacles 
between the master unit and slave unit.
 
The transmission distance differs according to the master unit used. 
Refer to the Instruction Manual of the device that is equipped with the 
master function.
 
Before shooting, perform a test flash (p.21) and test shooting.