Nikon D810 - Manual (Page 222)

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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 a camera control.
Select FV lock as the “press” option
for Custom Setting f4 (Assign Fn
button, 0 343), f5 (Assign preview
button, 0 349), or f6 (Assign AE-L/
AF-L button, 0 349).
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.
FV Lock
Flash pop-up button