Nikon D500 User Manual page 230

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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, 0 294). 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,
0 300). For information on automatically varying flash level over a
series of shots, see page 147.
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