Nikon D700 User Manual page 84

The camera's FX format (36.0 x 23.9 mm)
image sensor can be used to record pictures
with an image area (picture angle)
equivalent to a 35 mm (135) format film
camera. Image area is selected using the
Imagearea option in the shooting menu. At
the default setting of Auto DX crop, the camera will automatically
crop pictures to the DX picture angle when a DX format lens is
attached. The Choose image area option can be used to select a
35 mm picture angle.
IIAuto DXCrop
Choose whether to automatically select a DX crop when a DX lens
is attached.
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