Why do you think it is?
Originally Posted by Dan Fromm
You are probably thinking that the lens, at infinity, is at a distance from the film equal to the focal length. To get to 1:1, you need to add extension equal to the focal length. Thus twice as far away as when at infinity.
But unless the lens is a symmetrical thin lens, the lens is not (!) at a distance from the film equal to the focal length when set to infinity. So adding extension equal to the amount of the focal lenght does not (!) double the distance.
Hence your simple geometric assumption does not hold.
The rear focal length is measured from the rear principal plane. The thing important for light loss is the position of the exit pupil. The two are rarely found in the same spot.
So considerations concerning actual image geometry, i.e. magnification, do not tell us the important bit about bellows factors and light loss.
Last edited by Q.G.; 09-06-2010 at 11:59 AM. Click to view previous post history.