Quote Originally Posted by Q.G. View Post
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.
Yes. The inverse square law is still the thing; my point was that you can no longer use the attenuation = ((extension+focal length) / focal length) ^ 2 formula because the focal length is no longer a good approximation for the exit pupil distance. Does anyone here know the true position of the exit pupils of their non-normal lenses? I for sure don't and I'd have to be at about 6 sigmas for nerdiness and the gathering of otherwise-useless technical facts so I suspect most others don't either.

That makes it a little more difficult to calculate bellows factor for non-normal lenses, as I was saying.