The difference between fisheye lenses

Some people think that the focal length of fisheye lens must be smaller than that of super wide-angle lens, but it is not necessarily so. For example, Nikon ultra-wide-angle lenses have 13mm and 15mm, while fisheye lenses only have 16mm. The focal length of the fisheye lens of 135 camera is 6- 17mm, the focal length of the ultra-wide-angle lens is 13-24mm, and the focal length of the fisheye lens of 120 camera with 60x60 frames is 24mm. The focal length of the ultra-wide-angle lens is 35 mm, the focal length of the fisheye lens of the 60x70 frame 120 camera is 37mm, and the focal length of the ultra-wide-angle lens is 50 mm. The difference between fisheye lens and ultra-wide-angle lens lies not only in the length of focal length, but also in the size of viewing angle and whether to correct image distortion. For example, a super wide-angle lens with 13mm has a field of view of only 1 18, while a fisheye lens with 17mm. The visual field has reached 180. Ultra-wide-angle, like other lenses, tries to correct the distortion at the edge of the picture and try to make the picture consistent with the real thing. The fisheye lens intentionally retains the barrel distortion of the image to exaggerate its deformation effect. Except for the central part of the picture, all straight lines will become curved arcs. There are two basic types of fisheye lenses: one is a round fisheye lens, which is used to shoot a circular image with the frame width of the negative as the diameter. The other is a diagonal fisheye lens, which produces a rectangular distorted image with negative Quan Huafu.