Nikon D700 User Manual page 86

A
DX Lenses
DX lenses are designed for use with DX format cameras and have a smaller
picture angle than lenses for 35 mm format cameras. If Auto DX crop is
off and FX format (36 × 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
60
DX format (24 × 16)
image circle
FX format (36 × 24)
image circle