Canon Speedlite 90EX - Manual (Page 29)

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With manual flash and multiple Speedlites, you can set a different flash output for each slave unit.
All settings are made using the menus on the camera with the master unit.
1
Set the flash mode to <q>.
2
Select a firing group.
 Select < ALL >, or .
3
Set the flash output.
 Set the flash output for ALL, A, B, or C on the
menu.
Wireless Multiple Speedlite Shooting with Manual Flash Output
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