Nikon D5 - Manual (Page 224)

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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 (
0 330).
To use FV lock:
1
Assign FV lock to a camera control.
Assign FV lock to a control using Custom
Setting f1 (Custom control assignment,
0 307).
2
Attach a CLS-compatible flash unit.
Mount a CLS-compatible flash unit ( 0 330) 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
qA, or monitor pre-flash A. See the documentation
provided with the flash unit for details.
4
Focus.
Position the subject in the
center of the frame and press
the shutter-release button
halfway to focus.