Nikon D800 - Manual (Page 215)

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4
Lock flash level.
After confirming that the flash ready
indicator (
M) is displayed in the
viewfinder, press the Fn button. The
flash will emit a monitor preflash to
determine the appropriate flash level.
Flash output will be locked at this level
and FV lock icon (
e) will appear in the
viewfinder.
5
Recompose the photograph.
6
Take the photograph.
Press the shutter-release button the rest of the way down to
shoot. If desired, additional pictures can be taken without
releasing FV lock.
7
Release FV lock.
Press the Fn button to release FV lock. Confirm that the FV lock
icon (
e) is no longer displayed in the viewfinder.
Fn button