Even under bright sunlight, a flashgun can be useful for filling in shadows around the eyes and under the nose – but the shutter speed you need for a correct exposure might exceed the flash sync speed of the camera (typically around 1/200 sec). This is because the flash needs to occur after the shutter’s first curtain has fully opened, and before the second ‘rear’ curtain begins to close. To fix this, the Auto FP custom setting option in many Nikons fires a series of pulses of light throughout the exposure.
Available in your Nikon’s Custom Setting menu, Auto FP makes less maximum flash output available, but is great for filling in shadows.