Canon EOS 600D Instruction Manual page 104

Custom Wireless Flash Shooting
Fully-automatic Shooting with Multiple External Speedlites
Multiple Speedlite slave units can be treated as one flash unit or
separated into slave groups whose flash ratio can be set.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups with multiple
Speedlites.
Basic settings:
Flash mode : E=TTL II
E=TTL II meter, : Evaluative
Wireless func. : _=_
Channel : (Same as slave units)
[=_AII] Using multiple slave Speedlites as one flash unit
Convenient when you need a large flash
output. All the slave Speedlites will fire at
the same output and be controlled to
obtain a standard exposure.
No matter what the slave ID is (A, B, or
C), all the slave units will fire as one
group.
Set [Firing group] to [=_AII], then
take the picture.
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