Nikon D750 DSLR Camera (Body Only) - Manual (Page 219)

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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
icons (
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.