This article is a general guide and introduction to fisheye lenses, in which the topics about fisheye lenses are discussed, including the characteristic of fisheye lenses, the difference between fisheye lenses and wide angle lenses, different types of fisheye leneses, the advantages and disadvnatges of fisheye lenses, the design principle and structure of fisheye lenses, fisheye projection, etc. offering an in-depth explanation of fisheye lenses.