A fisheye lens is used in photography and cinematography and is a wide-angled lens that captures a broad, panoramic and hemispherical images. It has become popular as it provides a unique, distorted appearance. Different lenses distort images differently. There are two types of fisheye lens: circular or full-frame. Circular lenses take in a 180 degree hemisphere which is projected as a circle in the film frame. The corners of the frame will be dark when this lens is used.
The full frame lens is where; the picture angle produced by these lenses only measures 180 degrees when measured from corner to corner: these have a 180° diagonal angle of view, while the horizontal and vertical angles of view will be smaller. WIKIPEDIA
As technology advances certain images can now be digitally changed to become a full-frame fisheye image without the need for the special lenses themselves.