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.
Crop Factor
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.
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