Nikon D6 Reference Manual page 707

FV Lock
This feature is used to lock flash output for CLS-compatible flash
units, letting you take multiple photos or recompose shots
without changing the flash level. Your subject need not be in
the center of the frame, giving you more freedom when
composing shots.
•
Flash output is adjusted automatically for any changes in ISO
sensitivity and aperture.
1
Assign [FV lock] to a control
using Custom Setting f3
[Custom controls].
2
Mount a CLS-compatible flash unit on the camera
accessory shoe.
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FV Lock