Canon EOS-1D X Mark III Af Setting Manualbook page 123

How do you turn the flash firing
on or off quickly?
It is possible by assigning [Flash firing] to
any button by customizing the operation
buttons.
You can prevent the flash firing or
make it fire with an assigned button.
If you assign [Flash firing] to a specific button in the Custom
Controls, the flash will turn on or off each time you press the
button.
This setting is effective as it means you do not have to
take your eye away from the viewfinder to turn the flash
firing on or off. Assign [Flash firing] to a button that can
be assigned in the Custom Controls. With the flash enabled,
you can turn the flash on or off by pressing the assigned
button.
Assignable buttons
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button
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button
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button
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button
button