The angle of view of a lens changes according to the size of the sensor in the camera. A smaller sensor captures a narrower angle of view and makes it look as if the lens has a longer focal length. So in addition to the actual focal length, the manufacturers will usually quote the ‘effective’ focal length too.
This is the physical size of the sensor in a digital camera, which is independent of the number of megapixels it has. Bigger sensors capture more light and produce sharper, clearer images with less noise. In fact sensor size is the single most important factor these days in a camera's picture quality – megapixels are mostly secondary. Cameras with bigger sensors cost more, they're physically bigger and the lenses are bigger too.
See below for more articles about sensor size.
This is Nikon’s name for its APS-C format DSLRs. Some Nikon lenses are designed specifically for these smaller format models, and they include ‘DX’ in the lens name to signify that the can’t be used on the full frame models (well, they can, but only in a ‘DX crop’ mode.
Used to work out the effective focal length of lenses on cameras which don’t have full frame sensors. You multiply the actual focal length by the crop factor to get the effective focal length. The crop factor of an APS-C camera is 1.5, so a 50mm lens has an effective focal length of 75mm.
This is a relatively uncommon sensor size mid-way between APS-C and full frame. Canon used it for its EOS-1D high-speed pro sports/press photography DSLRs before these were merged with the introduction of the full frame EOS-1D X. Canon has since announced the development of a 250MP APS-H format sensor, though this has not yet been used in any commercial product. Sigma, meanwhile, has announced a new Sigma SD Quattro H mirrorless camera with a new APS-H format Foveon sensor. APS-H sensors measure approximately 30 x 20mm, or a couple of millimetres less.
This is the most common sensor size in cameras designed for enthusiasts and experts and it’s found in consumer DSLRs, mirrorless compact system cameras and some high-end compacts. APS-C sensors are around half the size of a full-frame sensor or the 35mm negative, and measure approximately 24 x 16mm. They have a crop factor of 1.5x, which means that you have to multiply the lens’s focal length by 1.5x to get its effective focal length in 35mm/full frame camera terms.