A fisheye lens is an ultra-wide-angle lens with an angle of view that is usually between 100 and 180 degrees and produces strong visual distortion intended to create a wide panoramic or hemispherical image.
Unlike wide angle and ultra wide angle lenses, a fish eye is an uncorrected lens so images produced with them have a characteristic convex non-rectilinear appearance.
For beginners it is easier to remember it this way; while using fish eye lenses; lines going around the center point are going to curve. And the further they are from the center point the more that they curve.
Circular Fisheye Lenses
|Circular Fisheye Lens - Photo Courtesy Piero Fissore|
A circular fisheye lens produces an image which is 180 degrees in all directions. Meaning it sees the world as a 180 degree hemisphere and projects the same as a circle on to the sensor and as a result you will have an image which has a 180 degree vertical angle of view, and 180 degree horizontal angle of view. As a result the corners of the frame will appear completely dark.
Full Frame Fisheye Lenses
|Full Frame Fisheye Lens - Photo Courtesy Tom Bricker|
These are improved versions of the above that fills the frame with image area. Unlike circular fisheye lenses, full frame fisheye lenses enlarge the image circle to cover the entire rectangular frame. The angle of view of pictures produced by these lenses measures 180 degrees when measured from corner to corner. They have a 180° diagonal angle of view, but the horizontal and vertical angles of view will be smaller.
For the 35 mm film format (full frame cameras), typical focal lengths of fisheye lenses are between 8 mm and 10 mm for circular images, and 15–16 mm for full-frame images.