Canon EOS-1D X Mark III DSLR Camera (Body Only) - Manual (Page 209)

Flash Custom Function Settings
For details on the Speedlite's Custom Functions, refer to the Instruction Manual of the
Speedlite.
1.
Select [Flash C.Fn settings].
2.
Set the desired functions.
Select the number.
Select an item.
Caution
With an EX series Speedlite, the Speedlite will always fire at full output if the [ Flash
metering mode ] Custom Function is set to [ TTL flash metering ] (autoflash).
The Speedlite's Personal Function (P.Fn) cannot be set or canceled from the
camera's [
: External Speedlite control ] screen. Set it directly on the Speedlite.
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