Nikon D810 - Manual (Page 223)

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4
Lock flash level.
After confirming that the flash ready
indicator (M) is displayed in the viewfinder, press the button
selected in Step 1. 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 button selected in Step 1 to release FV lock.
Confirm that the FV lock icon (e) is no longer displayed in the
viewfinder.