A cropped frame or sensor is similar to taking the middle of the image and discarding the outside edges. So essentially, you’re left with a slightly thinner image than normal — similar in shape to the short-lived APS film format. In fact, Canon, Pentax, and Sony usually refer to their cropped sensors as “APS-C” cameras. Just to confuse matters though, Nikon does things differently. Nikon’s full-frame cameras go under the moniker of “FX,” while its cropped frame cameras are known as “DX.” Finally, Olympus and Panasonic/Leica use a slightly different cropped format known as the Four Thirds system.
The crop of the sensor varies a little between manufacturers as well. Most manufacturers’ crop is smaller than a full frame sensor by a 1.6 ratio. However, Nikon’s ratio is 1.5 and Olympus’ ratio is 2.