Canon MR-14EX - Manual (Page 48)

Using the Wireless Flash System
Wireless Slave Groups
When C.Fn-5-1 Is set, you can use additional Speedlites and set their slave
IDs to A, B, or C.(^p.64)
By having multiple Speedlites set to the same slave ID, you can create a
slave group to produce a brighter flash. For example, a slave unit whose
slave ID is A will fire together with flash tube A. Each slave group Is treated
as one Speedllte, with the same flash settings applied to all the Speedlites
in the group.
There is no limit as to the number of slave units that can belong to one group.
With the EOS-1 D, IV, 3, D30, ELAN 7/7E, or 30/33, E-TTL autoflash control
can be applied individually to each of the three A, B, and C slave groups.
With other Type-A EOS cameras, the same E-TTL autoflash control is
applied to all three groups. In the manual flash mode, all Type-A EOS
cameras can apply E-TTL autoflash control individually to each of the three
slave groups.
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