Canon EOS REBEL T8i Advanced User's Manual page 226

External Flash Custom Function Settings
For details on the external Speedlite's Custom Functions, refer to the Speedlite's Instruction
Manual.
Caution
With an EL/EX series Speedlite, the Speedlite will always fire at full output if the
[Flash metering mode] Custom Function is set to [1:TTL] (autoflash).
The external Speedlite's Personal Function (P.Fn) cannot be set or canceled from
the camera's [
external Speedlite.
: External flash func. setting] screen. Set it directly on the
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