Nikon D810 User Manual page 221

A
Optional Flash Units
The flash compensation selected with the optional flash unit is added
to the flash compensation selected with the camera.
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See Also
For information on choosing the size of the increments available for
flash compensation, see Custom Setting b3 (Exp./flash comp. step
value, 0 315). For information on choosing how flash and exposure
compensation combine, see Custom Setting e4 (Exposure comp. for
flash, 0 338). For information on automatically varying flash level
over a series of shots, see page 133.
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