Quote Originally Posted by polyglot View Post
You are entirely correct for the symmetric thin lens approximation. If there are asymmetric lenses (e.g. with a pupil magnification ratio other than 1, i.e. telephoto or retrofocus lenses) then it gets a little messier because the lens when focused at infinity is not its focal length away from the film plane.
Edit: more importantly, its exit pupil is not the focal length away from the film.
That's true, insofar as that the focal length isn't important at all. It's always the distance between exit pupil and film that matters.
But that doesn't really make it messier: the inverse square law is the thing for hypothetical thin lenses, symmetric lenses, asymmetric lenses, etc.