Nikon D3S - Manual (Page 222)

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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.
To use FV lock:
1
Assign FV lock to the Fn button.
Select FV lock for Custom Setting f4
(Assign Fn button > Fn button press,
0 316).
2
Attach a CLS-compatible flash unit.
Mount an SB-900, SB-800, SB-600, SB-400, or SU-800 on the
camera accessory shoe.
3
Set the flash unit to TTL or AA mode.
Turn the flash unit on and set the flash mode to TTL or AA. See
the Speedlight instruction manual for details.