In digital cameras, the camera’s format refers to the size of its image sensor. Nikon makes a DX-format sensor and an FX-format sensor. The DX-format is the smaller sensor at 24x16mm; the larger FX-format sensor measures 36x24mm which is approximately the same size as 35mm film
Different NIKKOR lenses are designed to accommodate the different camera sensor sizes. In the case of DX cameras with their smaller sensors, corresponding DX lenses have been designed. The DX designation can be found in the lens name, i.e. AF-S DX Zoom-NIKKOR 12-24mm f/4G IF-ED. These lenses are smaller and lighter in weight and address the market’s need for affordable, high-performance lenses with a variety of focal lengths and zoom options.
The FX sensor, with more “light gathering” area, offers higher sensitivity and, generally, lower noise. There is, of course, no crop factor present with the FX sensor.
On an FX-format camera with a DX lens mounted, the camera will automatically engage its built-in DX crop mode, thus recording an image only from the center section of the sensor.