Nikon D3X User Manual page 88

A
DX Lenses
DX lenses are designed for use with DX format cameras and have a smaller
picture angle than lenses for 35mm format cameras. If [Auto DX crop] is
off and [FX format (36 × 24)] or [5 :4 (30 × 24)] is selected for [Image area]
when a DX lens is attached, the edges of the image may be eclipsed. This
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.
d
DX format
FX format
A
The Viewfinder Display
The DX format and 5 :4 crops are shown below.
62
5 : 4
DX format
DX format (24 × 16)
image circle
FX format (36 × 24)
image circle
5 : 4