Nikon D800 - Manual (Page 213)

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Normal flash output can be restored by setting flash
compensation to ±0.0. Flash compensation is not reset when the
camera is turned off.
A
Optional Flash Units
The SB-910, SB-900, SB-800, SB-700, and SB-600 also allow flash
compensation to be set using the controls on the flash unit; the value
selected with the flash unit is added to the value selected with the
camera.
A
See Also
For information on choosing the size of the increments available for
flash compensation, see Custom Setting b3 (Exp./flash comp. step
value,
0 287). For information on automatically varying flash level over
a series of shots, see page 132.