Nikon D780 DSLR Camera (Body Only) - Manual (Page 179)

133 Shooting Settings
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[Auto DX Crop]
Choose whether the camera automatically selects the [ DX
(24×16) ] (DX format) image area when a DX lens is attached.
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The Viewfinder Mask Display
If [ On ] is selected, the area outside crop ([ FX (36×24) ] excepted)
will be shown in gray in the viewfinder.
Option Description
[ On ]
The camera automatically selects the [ DX (24×16) ]
(DX format) image area when a DX lens is attached.
[ Off ] Automatic image-area selection disabled.