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
Angle or "terminal side" is defined by the measure from the initial side in radians, degrees, or turns. With "positive angles" representing rotations toward the positive y-axis and "negative angles" representing rotations toward the negative "y"-axis. When Cartesian coordinates are represented by "standard position", defined by the "x"-axis rightward and the "y"-axis upward, positive rotations are anticlockwise and negative rotations are clockwise. In many contexts, an angle of −"θ" is effectively equivalent to an angle of "one full turn minus "θ"". For example, an orientation represented as −45° is effectively equivalent to an orientation represented as 360° − 45° or 315°.