>>3786491Optics are quite complex. What affects focal length is the distance between the point of convergence/focal point and the image sensor or film plane. Since optics work by bending light (refraction) you can manipulate this focal point to be inside the lens or outside of the lens through use of various combinations of lens element shapes. There are a great many different optical arrangements that can be used to achieve this with different pros and cons and they are all well beyond me with my only basic optical knowledge from my schooling.
