Canon EOS 80D Instruction Manual page 250

Wireless Flash PhotographyN
Fully Automatic Shooting with Multiple External Speedlites
Multiple Speedlite slave units can be fired as one flash unit or separated
into slave groups for shooting with flash ratio control.
The basic settings are shown below. By changing the [Firing group]
setting, you can shoot with various wireless flash setups with multiple
Speedlites.
[1All] Firing multiple slave Speedlites as one flash unit
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250
Basic settings:
Flash mode
Wireless func. :0
Channel
Effective 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 which firing group (A, B, or C)
the slave units belong to, they will all fire
as one group.
Set [Firing group] to [1All].
1
Take the picture.
2
: E-TTL II
: (Same as slave units)