Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 186)

186
Wireless Flash Photography Using Optical Transmission
 [
1
(A:B)] Autofl ash with two receiver groups
A B
Receivers can be assigned as fl ash
groups A and B, and you can adjust the
relative output of each.
Exposure is automatically controlled,
so that the groups’ combined output
provides standard exposure.
1 Set [Firing group] to [ 1 (A:B)].
2 Set [A:B fi re ratio].
o Flash units in group C will not fi re when [Flash group] is set to [
1
(A:B)] .
o Flash ratios from 8:1 to 1:1 correspond to 3:1 to 1:1 (in 1/2-stop increments)
when converted to number of stops of exposure.