Optics technical overview
The focal length of a lens is the distance between its effective centre and the point at which light from infinity will be focused to a single point. The sensor of the camera needs to be positioned behind this point on what is called the image plane.
Field of view or magnification
The magnification ß' of a lens describes the ratio between image size and object size and has a direct connection to the focal length f' of the lens and the working distance a. The following diagram shows how to calculate the ratio between object size, working distance and focal length of the lens.