Fisheye lens

A fisheye lens (or fish eye lens) is a wide-angle lens, which has an very wide angle of view. Unlike normal wide angle lenses, fisheye lernses use Gnomonic projection, which will distort the image. Straight lines which do not run through the center of the image will be shown as curved ones. Common fisheye lenses have angles of view between 180 and 220 degrees. Prototypes have been built with angles of view as large as 270 and even 310 degrees. Due to their construction, conventional wide angle lenses cannot reach angles of view greater than 180 degrees. Even though they have very wide angles of view, fisheye lenses show less vignetting. Vignetting occurs when the border of the image is much darker than the center. 
Because of their wide angle of view, fisheye lenses are often used in cameras used for surveillance.