Nikon D300 - Manual (Page 204)

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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.
To use FV lock:
1
Assign FV lock to the Fn button.
Select [FV lock] for Custom Setting f4
([Assign FUNC. button] > [FUNC. button
press], pg. 302).
2
Press the flash pop-up button.
The built-in flash will pop up and begin
charging.
3
Focus.
Position the subject in the
center of the frame and press
the shutter-release button
halfway to focus.
Flash pop-up
button