Image sensor sizes & crop factors
Shallow DoF is often desirable in order to provide increased emphasis on the primary subject in a scene by defocusing secondary elements. The DoF decreases (becomes shallower) as the lens' focal length is increased and aperture becomes wider (lower f / T stop). The larger the image sensor, the shallower the achieved DoF will be at a given focal length and aperture.
The following graphic illustrates various sensor sizes when compared to Super 35mm film and legacy video camera sensors (The sensor illustrations are actual size when viewed at 72dpi):
Lenses designed for use on a 35mm full frame camera project an image onto an area of 36 x 24mm at the focal plane, so when a smaller sensor is used at the focal plane a crop factor is applied because the smaller sensor gives a zoom-in effect. Crop factors for various common sensor sized are illustrated above. As an example: If a 35mm SLR lens lens with a 50mm focal length is used on a camera with a 4/3 sensor then a crop factor of 2 applies and the lens appears to have an effective focal length of 100mm when compared to the same lens in use on a 35mm full-frame camera.