Nikon D750 DSLR Camera (Body Only) - Manual (Page 119)

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A : Aperture-Priority Auto
In aperture-priority auto, you choose the aperture while the
camera automatically selects the shutter speed that will produce
the optimal exposure.
To choose an aperture between the
minimum and maximum values for the
lens, rotate the sub-command dial while
the exposure meters are on.
Sub-command dial
Control panel