APS-C Lens Formats
Canon, Nikon, Pentax and Sony have developed and designed lenses specifically for their cameras with a lens factor (more fully, lens focal length conversion factor) or "crop factor". APS-C cameras use a smaller area to form the image than traditional 35 mm cameras, and so lenses used on APS-C format cameras have a correspondingly narrower field of view. For example, a 28 mm lens is a wide angle lens on a traditional 35mm camera. But the same lens on an APS-C camera, with a lens factor of 1.6x (relative to a standard full-frame 35mm format camera), has the same angle of view as a 45 mm (28 mm x 1.6 lens factor) lens on a 35 mm camera—i.e. a normal lens. Several third-party lens manufacturers, such as Tamron, Tokina, and Sigma, also manufacture a range of lenses optimised for APS-C sensors.
Canon introduced the Canon EF-S line of lenses in 2003 alongside the 300D. These lenses place the rear of the lens closer to the camera's sensor (referred to as short back focus). This has several benefits, including lighter lenses and a narrower field of view (which implies “longer” zoom). EF-S lenses are compatible with Canon's APS-C digital SLRs, with the exception of the early Canon EOS D30, Canon EOS D60, and Canon EOS 10D, which predated the introduction of the mounting system. EF-S lenses will not physically mount on Canon's full-frame digital or 35mm film SLRs.
Nikon has their DX format for their line of APS-C digital cameras. These can be mounted to all full-frame Nikon digital bodies at the cost of fewer megapixels. These lenses generally exhibit vignetting when mounted on Nikon film bodies, but may be usable at longer focal lengths.
Pentax produces the DA line for their APS-C cameras (the company has yet to introduce a full-frame DSLR). These lenses are available in focal lengths that offer similar field-of-view as lenses previously available for 135 film. The trademark compact design of the DA limited series takes advantage of the smaller APS-C format with the lenses under 40mm and is fully usable on 135 Film with the DA Limited Lenses over 35mm focal length. All DA lenses can be mounted on Pentax film bodies, albeit with increased vignetting. All fixed focal lengths in the rugged DA* series cover 35 film format fully.
Sony has a DT line specifically designed for their APS-C cameras. These lenses can be mounted on any Sony Alpha, but are specifically designed for the A100-A700 series of DSLRs, the earlier Konica Minolta Dynax 5D and 7D, and the current Alpha SLTs except for the full-frame A99. DT lenses can be mounted on an A850, A900, or A99 in "crop" mode, where the frame is cropped and the resolution is roughly halved.
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