What is the function of each lens in the fisheye lens group?

A fisheye lens is a lens with a focal length of 16mm or shorter and a viewing angle close to or equal to180. It is the extreme wide-angle lens with the highest viewing angle among many lenses. In order to make the lens reach the maximum shooting angle, the front lens of this kind of photographic lens has a short diameter and protrudes to the front of the lens in a parabolic shape, which is quite similar to the fish's eyes, hence the name "fisheye lens". Fisheye lens belongs to a special super wide-angle lens, and its viewing angle strives to reach or exceed the range that human eyes can see. Therefore, the fisheye lens is very different from the real world scene in people's eyes, because the scenery we see in real life is regular and fixed, and the picture effect produced through the fisheye lens is beyond this category. Many photographers like to use exaggerated and distorted fisheye glasses to create a sense of perspective. The widest fisheye lens in history is 6 mm f/2.8 owned by Yikang, with a viewing angle close to 220. The biggest function of fisheye lens is its wide viewing angle, which can generally reach 220 or 230, creating conditions for shooting large-scale scenes at close range; The fisheye lens can produce a very strong perspective effect when shooting close to the subject, emphasizing the contrast between the near big and the far small of the subject, so that the shot picture has a shocking appeal; The fisheye lens has a large depth of field, which is beneficial to show the long depth of field effect of photos. There are two kinds of fisheye lens imaging, one is full of pictures like other lenses; The other imaging is circular. No matter what kind of imaging, the images taken with fisheye lens are quite deformed and have a strong sense of perspective convergence. Therefore, it is often used as a special effect lens, and an arm extending to the fisheye lens will appear twice as long as the original one. For example, Wong Kar-wai's fallen angels (1995) used a lot of fisheye lenses. As we all know, the shorter the focal length, the larger the viewing angle and the stronger the deformation caused by the optical principle. In order to realize the super-large viewing angle of 180 degrees, the designers of fisheye lenses have to make sacrifices, that is, to allow this deformation (barrel distortion) to exist reasonably. In this way, except for the scene in the center of the picture, other scenes that should be horizontal or vertical have changed accordingly. It is this strong visual effect that provides an opportunity for imaginative and challenging photographers to show their personal creativity.