Canon EOS 90D DSLR Camera with 18-135mm Lens - Manual (Page 168)

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Flash Function Settings
Functions of the built-in fl ash or external EL/EX series Speedlites can be set
from menu screens on the camera.
Before setting functions of external Speedlites, attach the Speedlite and turn
it on.
For details on external Speedlite functions, refer to the Speedlite’s
instruction manual.
1 Select [ z : Flash control].
2 Select an item.