Canon Speedlite 600EX-RT Instruction Manual page 113

Optical Transmission Wireless Shooting
The slave unit does not fire.
Set the master unit to <:> <M> and set the slave unit to
<:> <x> (p.78).
Set the transmission channels of the master unit and slave unit to the
same numbers (p.78).
Check that the slave unit is within the transmission range of the master
unit (p.76).
Point the wireless sensor on the slave unit toward the master unit
(p.76).
If the master unit and slave unit are too close, the transmission may not
operate properly.
When using the camera's built-in flash as the master unit, raise the
camera's built-in flash, and set the wireless function in [Built-in flash
func. setting] on the camera's menu screen.
The master unit's flash fires.
Even when master flash firing is set to OFF <d>, the master flash fires
a small flash to control the slave unit with optical transmission (p.79).
Pictures are overexposed.
When using autoflash shooting with three firing groups A, B and C, do
not fire with firing group C pointed toward the main subject (p.87).
Troubleshooting Guide
111