Canon EOS Rebel T5i 18-55mm IS STM Lens Kit - Manual (Page 235)

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Custom Wireless Flash Shooting
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. :0
Channel
: (Same as slave units)
[1All] Using multiple slave Speedlites as one flash unit
This is convenient when you need a
large flash output. All the slave units 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 [1All], then
take the picture.
Fully Automatic Shooting with Multiple External Speedlites
COP
Y