Nikon D3 and D700 can switch between DX (24x16mm) and FX (36x24mm) modes to accommodate different lens formats. If “Auto DX Crop” is enabled, the camera will automatically adjust the picture angle (field of view) to DX format when a DX lens is attached, and to FX format if a 35mm format lens is attached.
Obviously, the DX format can use only the pixels in the center of the FX frame on the FX DSLRs. For Nikon D3 and D700, it is 5MP. This is indeed a lot fewer pixels compared to a DX format DSLR such as Nikon D300, but you enjoy the superior picture quality throughout a wider ISO sensitivity range attributed to larger pixel size that yields a higher signal-to-noise ratio and wider dynamic range. Therefore users can actually take advantage of the DX mode when you only need or have the center area of the frame even with a FX format lens. Example of such situation is when your subject is too far away to fill the FX frame. Not only you have smaller file size to handle, but also you don’t need to crop every photo to remove unnecessary pixels.
To switch to DX mode, you will need to turn Auto DX crop off and select DX format by going to Shooting menu-> Image area. In DX format, the effective focal length will be 1.5x the focal length of the lens.
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Keywords: D3, D700, Digital SLR Cameras, DX, full frame, fx, Nikon, SLR Camera