Nikon D6 Reference Manual page 191

❚❚ Auto DX Crop
Choose whether the camera automatically selects the [DX
(24×16)] (DX format) image area when a DX lens is attached.
Option
The camera automatically selects the [DX (24×16)] (DX
[On]
format) image area when a DX lens is attached.
[Off] Automatic image-area selection disabled.
D
Cropping
DX lenses are designed for use with DX format cameras and have a
smaller angle of view than lenses for 35 mm format cameras. If [Off] is
selected for [Auto DX crop] and an option other than [DX (24 × 16)] (DX
format) is selected for [Choose image area] when a DX lens is attached,
the edges of the image may be eclipsed. This is referred to as "cropping".
Cropping may not be apparent in the viewfinder, but when the images
are played back you may notice a drop in resolution or that the edges of
the picture are blacked out.
Description
Image Recording Options (Image Area, Quality, and Size)
139