Canon EOS-1 D X Mark II Instruction Manual page 269

Clearing Flash Function Settings / Flash C.Fn Settings
The Speedlite's Personal Function (P.Fn) cannot be set or canceled on the
camera's [External Speedlite control] screen. Set it directly on the
Speedlite.
Select [Clear settings].
1
Select the settings to be cleared.
2
Select [Clear flash settings] or
[Clear all Speedlite C.Fn's], then
press <0>.
On the confirmation dialog, select
[OK]. Then the flash settings or
Custom Function settings will all be
cleared.
3 Setting the Flash Function
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