Nikon D780 DSLR Camera (Body Only) - Manual (Page 517)

471 On-Camera Flash Photography
This feature is used to lock flash output for CLS-compatible flash
units, allowing photographs to be recomposed without
changing the flash level and ensuring that flash output is
appropriate to the subject even when the subject is not
positioned in the center of the frame. Flash output is adjusted
automatically for any changes in ISO sensitivity and aperture. FV
lock is not available in
b and EFCT modes.
To use FV lock:
1
Assign [FV lock] to a camera
control.
Assign [ FV lock ] to a control using
Custom Setting f3 [ Custom
controls ].
2
Attach a CLS-compatible flash unit.
Mount a CLS-compatible flash unit on the camera accessory
shoe.
FV Lock