Ultra wide angle lens

An ultra wide-angle lens is a lens whose focal length is shorter than the short side of film or sensor.

Thus the term denotes a different range of lenses, relative to the size of the sensor in the camera in question.

Ultra-wide angle lenses come in two varieties: Fisheye lenses with curvilinear barrel distortion, and rectilinear lenses which are designed so that straight lines in the scene will render straight (uncurved) in the photographic image and thus lack the extreme distortion that is characteristic of a fisheye lens. Neither denotes a particular range of focal lengths, the difference is only whether distortion is present or not. However, the shorter the focal length, the more difficult it is to implement rectilinear correction.

In fisheye lenses, the visual angle is close to or more than 180 degrees in at least one direction. For example, a "diagonal fish eye" would have a viewing angle of at least 180 degrees within the diagonals of the frame. A "circular fisheye" would represent the image in the form of a circle.

Rectilinear ultra-wide angle lenses are used in photography and cinematography sometimes to achieve three-dimensional perspective distortion instead of simply two-dimensional barrel distortion. A notable, signature employment for this purpose is frequently seen in the films of Terry Gilliam, for instance.

The DOF afforded by an ultra wide-angle lens is very great. Therefore the photographer has the ability to keep much or almost all of the scene in focus, with respect to the hyperfocal distance of the lens.

Thanks to the small focal length, these lenses can shoot longer exposures without fear of camera shake in the image. (In longer lenses camera shake is multiplied by the zoom factor, but in shorter lenses it is much less apparent). This means that the photographer can afford to use a much smaller aperture if they choose, and still retain a balanced image.

With such a large visual angle, it can be very difficult at times to keep undesired objects such as light sources out of the image. However this does not always pose a problem, as even the sun in a photograph takes up such a small amount of space that its presence can often have little negative impact on the overall composition.

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Wikipedia