A fisheye lens is an ultra-wide-angle lens that has angle of view 180 degree or more. The design of such lenses and their shape are really to some extent similar to the organ of sight of fishes. Perspective distortion in such optical systems cannot be neutralized, and the lens forms a bizarrely distorted convex-rounded picture, which is the main distinguishing feature of the fisheye lenses.
In practice, there are two main types of such lenses:
1.Circular lenses, which give a circular image within the film frame, and thus they have the angle of field of view 180 degree in any direction.
2.Diagonal or full-frame lenses, the image circle of which is circumscribed around the film, so that only the diagonal angle of view is 180 degree.