Canon EOS Rebel T4i 18-135mm IS STM Lens Kit Instruction Manual page 229

Fully Automatic Shooting with Multiple External Speedlites
You can have multiple slave units fire as if they were a single Speedlite.
This is convenient when you need a large flash output.
Flash Exposure Compensation
If the flash exposure looks too dark or too bright, you can set flash
exposure compensation to adjust the slave units' flash output.
Basic settings:
Flash mode
E-TTL II meter. : Evaluative
Built-in flash
Channel
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.
Select [Flash exposure
compensation], then press <0>.
If the flash exposure is too dark, press the
<Z> key to increase the flash exposure
and make it brighter. If the flash exposure is
too bright, press the <Y> key to decrease
the flash exposure and make it darker.
Easy Wireless Flash Shooting
: E-TTL II
: EasyWireless
: (Same as slave units)
229