Ultra wide angle lens 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
Ultra wide angle lens 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,