Nikon D5 User Manual page 225

At values other than ±0.0, a Y icon will be displayed in the top
control panel and viewfinder after you release the W (M) button.
The current value for flash compensation can be confirmed by
pressing the W (M) button.
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
In i-TTL and auto aperture (qA) flash control modes, the flash
compensation selected with the optional flash unit or the Flash control
option in the photo shooting menu is added to the flash compensation
selected with the W (M) button and command dial.
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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, 0 331). For information on choosing whether flash
compensation is applied in addition to exposure compensation when
the flash is used, see Custom Setting e3 (Exposure comp. for flash,
0 342). For information on automatically varying flash level over a
series of shots, see page 147.
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