I wasn ' t listing children ' s support as evidence god is real . I ' m saying that they aren ' t necessarily brainwashed . The tooth fairy isn ' t shoved down kids throats , but the kids genuinely believe it . <| sep |> Nothing in your comment is correct . To measure the focal length of a lens , you use it to image an object at infinity . Where the cone of light going to the focal point intersects the extension of the rays entering the lens as if they had never gone into it meet is the focal length . For an object at infinity , the focal plane is by definition the film plane . This is not the middle of the lens . Take for example the 8 mm fisheye lenses available for DSLR cameras . The focal length is 8 mm , and the flange distance is ~ 45 mm . If you tried anything from the middle of the lens you would be off by nearly an order of magnitude . 40 mm ish SLR pancake lenses are not tessar designs . The rear principal point of a tessar is in the middle of the lens , which is around 50 mm from the film plane . They must necessarily then be retrofocus designs .