The diameter of a sphere is four times the focal length of a concave mirror cut from it.
凹面镜如果是从圆球上切下来的，圆球的直径会是凹面镜焦距的四倍。
2
The focal length of camera is estimated efficiently by using the algorithm of equidistant matching, images are warped to the surface of sphere whose radius equals to the focal length of camera.