Image Areas
The Nikon D3X is unusual in that you can not only use it as a full-frame SLR, but also as a reduced-frame one. In fact, there are two cropped-frame modes available, one with a 5:4 aspect ratio (corresponding to an area on the sensor of 30.0 x 23.9 mm), the other a 3:2 aspect ratio, but with the frame size reduced to match that of Nikon's DX-format cameras. (A 23.5 x 15.6 mm frame, vs the full "FX" format size of 36.0 x 23.9 mm.) While we wouldn't personally find the 5:4 ratio useful, it's quite possible that photographers working on specific assignments calling for a more square format would want to take advantage of that aspect ratio. The "DX" frame size is handy for taking advantage of Nikon's many excellent DX-format lenses. In fact, the camera can be set to automatically switch to DX mode when a DX-format lens is attached to it. You can rapidly select between crop modes by pressing and holding the Fn button on the front of the camera while simultaneously turning the main command dial on the rear. With a 24.5 megapixel sensor, the D3X has a more generous 10.5-megapixels available in DX mode (the D3 retained only 5.1-megapixels in DX mode).

When the D3X is shooting in a format other than full-frame FX mode, a translucent LCD in the viewfinder partially masks the inactive portions of the frame. The illustrations below show the viewfinder display in 5:4 and DX modes.

VF masked for 5:4 mode	VF masked for DX mode

AF Areas
Like its sibling the D3, the D3X features Nikon's Multi-CAM3500FX, a 51-point AF system. The fifteen points in the center of the frame (3 columns x 5 rows) act as cross-type sensors at apertures of f/5.6 and larger. This is quite a benefit for photographers needing to shoot with slower zoom lenses, or large teles on teleconverters. (Something that will more likely be required, given the lack of a crop factor when using the D3X in FX mode.) Strapping on Nikon's 1.4x teleconverter won't quite get you back to the same field of view as you'd have with the same lens on an APS-C body like the D300. - Many sports and nature photographers accustomed to the longer reach of their lenses on DX-format bodies are going to find themselves reaching for their teleconverters more frequently with the D3X.)

The Nikon D3X doesn't force you to use all 51 AF points if you don't want to though: Custom Menu option A8 lets you select between the full 51-point array and an 11-point arrangement that mimics the layout of the D2x's AF system. Shooters acquainted with the D2x may elect to use the 11-point AF layout out of familiarity, but we think most users will just stick with the full 51-point array. The illustrations below show the two AF arrays in the full FX-format viewfinder window.

Full 51-point AF array	Reduced 11-point AF layout