Canon Speedlite 90EX Instruction Manual page 25

Wireless Flash with Three Slave Groups
You can have slave groups A and B and also add slave group C. You can use slave groups
A and B to obtain the standard flash exposure of the subject, and slave group C to illuminate
the background to eliminate shadows.
C
A
If <l> is set, the slave unit in slave group <3> will not fire.
If you point the slave unit in slave group <3> toward the subject, the subject will be
overexposed.
1
Set the slave units.
See "Setting the Slave Units" on page 23 to set the
slave unit's ID to <1>, <2>, or <3>.
For slave <3>, also set the flash exposure
compensation as necessary.
B
2
Set the master unit and shoot.
Follow "Setting the Master Unit Flash Ratio" on
page 23. In step 1, select <k>.
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