Nikon D4 - Manual (Page 230)

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FV Lock
This feature is used to lock flash output, 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 available with CLS compatible flash units only (
0 192).
To use FV lock:
1
Assign FV lock to the Fn button.
Select FV lock for Custom Setting f3
(Assign Fn button > Press,
0 328).
2
Attach a CLS-compatible flash unit.
Mount a CLS-compatible flash unit ( 0 193) on the camera
accessory shoe.
3
Set the flash unit to the appropriate mode.
Turn the flash unit on and set the flash mode to TTL, monitor
pre-flash AA, or monitor pre-flash A. See the Speedlight
instruction manual for details.