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Cutaway Diagram of Wide Angle lens  

Copywright 2008, Idiomorf infographics  

Wide-angle lenses for 35 mm format
For a full-frame 35 mm camera with a 36 mm by 24 mm format, the diagonal measures 43.3 mm and by custom, the normal lens adopted by most manufacturers is 50 mm. Also by custom, a lens of focal length 35 mm or less is considered wide-angle. Common wide-angle lenses for a full-frame 35 mm camera are 35, 28, 24, 21, 18 and 14 mm. Many of the lenses in this range will produce a more or less rectilinear image at the film plane (though some degree of barrel distortion is not uncommon here). Extreme wide-angle lenses that do not produce a rectilinear image are called fisheye lenses. Common focal lengths for these in a 35 mm camera are 6 to 8 mm (which produce a circular image). Lenses with focal lengths of 14 to 16 mm may be either rectilinear or fisheye designs.
Wide-angle lenses come in both fixed-focal-length and zoom varieties. For 35 mm cameras, lenses producing rectilinear images can be found at focal lengths as short as 12 mm, including zoom lenses with ranges of 2:1 that also begin at 12 mm.

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Digital camera considerations Most interchangeable-lens digital cameras today (2008) are in the form of 35 mm cameras. However, most of these cameras have photosensors that are smaller than the image apertures of full-frame 35 mm cameras.[3] For the most part, the dimensions of these photosensors are similar to the APS-C image frame size, i.e., approximately 24 mm x 16 mm. Therefore, the angle of view for any given focal length lens will be narrower than it would be in a full-frame camera because the smaller sensor "sees" less of the image projected by the lens. The camera manufacturers provide a crop factor (sometimes called a field-of-view factor or a focal-length multiplier) to show how much smaller the sensor is than a full 35 mm film frame. For example, one common factor is 1.5 (Nikon DX format and some others), although many cameras have crop factors of 1.6 (most Canon DSLRs), 1.7 (the Sigma DSLRs) and 2 (the Four-thirds-format cameras). The 1.5 indicates that the angle of view of a lens on the camera is the same as a 35 mm full-frame camera with a focal length of 1.5 times the focal length, which explains why the crop factor is also known as a focal-length multiplier. As examples, a 28 mm lens would produce on the DSLR the angle of view of a 42 mm lens (given a crop factor of 1.5) on a full-frame camera. So, to determine the focal length of a lens for a digital camera that will give the equivalent angle of view as one on a full-frame camera, the full-frame lens focal length must be divided by the crop factor. For example, to get the equivalent angle of view of a 28 mm lens on a full-frame 35 mm camera, from a digital camera with a 1.5 crop factor, one would use an 18 mm[4] lens.
Lens manufacturers have responded to this problem by making wide-angle lenses of much shorter focal lengths for these cameras. In doing this, they limit the diameter of the image projected to slightly more than the diagonal measurement of the photosensor. This gives the designers more flexibility in providing the optical corrections necessary to economically produce high quality images at these short focal lengths, especially when the lenses are zoom lenses. Examples are 10 mm minimum focal length zoom lenses from several manufacturers. At 10 mm, these lenses provide the angle of view of a 15 mm lens on a full-frame camera when the crop factor is 1.5.

Cross-section of a typical short-focus wide-angle lens.
There are two different varieties of wide-angle lens: short-focus lenses and retrofocus lenses. Short-focus lenses are generally made up of multiple glass elements whose shapes are more or less symmetrical in front of and behind the diaphragm. As the focal length decreases, the distance of the rear element of the lens from the film plane or digital sensor also decreases. This makes short-focus wide-angle lenses undesirable for single-lens reflex cameras unless they are used with the reflex mirrors locked up. Short-focus lenses are widely used on large format view cameras. The retrofocus lens solves this proximity problem through an asymmetrical design that allows the rear element to be further away from the film plane than its effective focal length would suggest. (See Angenieux retrofocus.) For example, it is not uncommon for the rear element of a retrofocus lens of 18 mm to be more than 25 mm from the film plane. This makes it possible to design wide-angle lenses for single-lens reflex cameras.
One of the disadvantages of small sensor APS-C Digital SLR cameras (basically everything but the Canon 1D and 1Ds series cameras) is that the 1.6x  "multiplier/crop" factor makes all your existing 35mm wide-angle lenses into "not so wide" angle lenses. Nikon cameras are slightly better, with a 1.5x "multiplier/crop", but still basically suffer from the same problem. On a Canon 20D or Canon Digital Rebel XT your 24mm lens has the same field of view as a 38mm lens would have on a full frame camera. Your wide 20mm becomes a moderate 32mm and even your superwide 16-35 zoom becomes a mid-range 26-56 lens. Even the widest rectilinear prime lens you can get for 35mm a 14mm - turns into only a 22.5mm lens.

S o what can you do? Well , you have to turn to one of the new ultrawide zooms made specifically for small (APS-C) sensor digitalcameras. While Nikon and Canon have had their own wide-angle lenses for a while, there are now also four "3rd party" alternatives available.

For now here is a comparison of the Ultrawide zooms available for Canon and Nikon APS-C sensor digital cameras. I believe the 3rd party lenses are also available in a Konica-Minolta mount.