Canon EOS 90D Advanced User's Manual page 177

 Flash exposure bracketing
External Flash Custom Function Settings
With an EL/EX series Speedlite, the Speedlite will always fi re at full output if
o
the [Flash metering mode] Custom Function is set to [TTL fl ash metering]
(autofl ash).
The external Speedlite's Personal Function (P.Fn) cannot be set or canceled
o
from the camera's [ z : External Speedlite control] screen. Set it directly on
the external Speedlite.
External Speedlites equipped with fl ash
exposure bracketing (FEB) can change the
external fl ash output automatically as three
shots are taken at once.
For details on the external Speedlite's
Custom Functions, refer to the Speedlite's
Instruction Manual.
Flash Function Settings
177