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
Back-in angle parking in traffic delays and considerable risk of collisions with pedestrians, bicyclists, and vehicles. Existing pull-in angle parking spaces can be converted to back-in angle parking by re-striping to flip the angle of the stall markings. Streets with parallel parking have also been converted to back-in angle parking. In either case, the pavement markings are often augmented with signage indicating that vehicles must be backed in. According to the Pedestrian & Bicycle Information Center, back-in angle parking provides motorists with better vision of pedestrians, bicyclists, motor vehicles, and other road users as they exit a parking space and enter moving traffic.