Nikon D500 - Manual (Page 232)

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5
Lock flash level.
After confirming that the
flash-ready indicator (
M) is displayed in the viewfinder, press
the control 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.
6
Recompose the photograph.
7
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.
8
Release FV lock.
Press the control selected in Step 1 to release FV lock.
Confirm that the FV lock icon (
e) is no longer displayed in the
viewfinder.