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
Straight razor as heavy stropping and honing. Shaving is done with the blade at approximately an angle of thirty degrees to the skin and in a direction perpendicular to the edge; an incision requires the movement of the blade to be sideways or in a direction parallel to the edge. These circumstances are always avoided by the shaver, who always shaves in a direction perpendicular to the cutting edge of the blade. To be most effective, a straight razor must be kept extremely sharp. The edge is delicate, and inexpert use may bend or fold over the razor's edge. To unfold and