Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 269)

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3 Setting the Flash Function
1
Select [Clear settings].
2
Select the settings to be cleared.
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.
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.