In photography, a fisheye lens is a wide-angle lens that projects deliberately distorted images.
All ultra-wide angle lenses suffer from some amount of distortion. While this can easily be corrected for moderately wide angles of view, rectilinear ultra-wide angle lenses with angles of view greater than 90 degrees are difficult to design. Fisheye lenses achieve extremely wide angles of view by foregoing a rectilinear image, opting instead for a cylindrical perspective, which gives images a characteristic convex appearance.
A special kind of fisheye lens, called circular fisheye lens, projects a circular image fully contained inside the frame, typically with an angle of view of 180 degrees.
The focal lengths of fisheye lenses depend on the film format. For the popular 35mm film format, typical focal lengths of fisheye lenses range from 6mm to 16mm.