Ultra Wide Angles

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About Ultra Wide Angles

lenses displaying convergance..

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.

[edit] Digital camera considerations
Apparent focal length in APS-sized digital cameras is increased by a crop factor.
Apparent focal length in APS-sized digital cameras is increased by a crop factor.

Most interchangeable-lens digital cameras today (2007) 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 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.