Nikon D800 - Manual (Page 214)

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FV Lock
This feature is used to lock flash output, allowing photographs to
be recomposed without changing the flash level and ensuring that
flash output is appropriate to the subject even when the subject is
not positioned in the center of the frame. Flash output is adjusted
automatically for any changes in ISO sensitivity and aperture.
To use FV lock:
1
Assign FV lock to the Fn button.
Select FV lock for Custom Setting f4
(Assign Fn button > Fn button press,
0 311).
2
Press the flash pop-up button.
The built-in flash will pop up and begin
charging.
3
Focus.
Position the subject in the
center of the frame and press
the shutter-release button
halfway to focus.
Flash pop-up button