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
Antiparallel (mathematics) and only if they make the same angle formula_18 in the opposite senses with the bisector of that angle (Fig.3). In a Euclidean space, two directed line segments, often called "vectors" in applied mathematics, are antiparallel, if they are supported by parallel lines and have opposite directions. In that case, one of the associated Euclidean vectors is the product of the other by a negative number. Antiparallel (mathematics) In geometry, anti-parallel lines can be defined with respect to either lines or angles. Given two lines formula_1 and formula_2, lines formula_3 and formula_4 are anti-parallel with respect to formula_1 and formula_2