On 9 July 2012 03:25, Mark C <pdml-m...@charter.net> wrote: > I have never found a good explanation of what is going on when you reverse > mount a lens. After using reverse mounted lenses quite a bit, I can say that > reversing the lens allows you to focus closer. It also seems like the > subject to lens distance does not change as much as you'd expect when you > reverse mount and change extension. But I've never learned the theory.
If the lens is a symmetrical design without any floating or FREE groups of elements then the formula works equally whether the lens is mounted normally or reversed. The purpose of reverse mounting in this case is not to gain magnification, because it won't. The theory behind it is that a lens in regular, non-macro use is closer to the focal plane than the subject plane and is best corrected for that circumstance. When you exceed 1:1 magnification the lens becomes closer to the subject plane than the focal plane, and gets out of the range for which it is best corrected. Reverse mounting it goes some way towards restoring its balance. That is, it makes the plane of focus on the rear element side nearer than the plane of focus on the front element side, as it should be. There's no cover-all formula for reverse-mounted non-symmetrical lenses. A telephoto lens is almost useless reversed. Imagine putting a negative dioptre filter in front of your lens. It defeats the purpose of photomacrography and is often impossible because the subject plane falls inside the lens. But reverse mounted wide-angle lenses are famously effective. Keep in mind that, at least in SLR mounts, they were originally known as inverted telephoto lenses and are best corrected for large reductions. Reverse mount one of those and you have a true telephoto (but still of short focal length) that's best corrected for large magnifications. regards, Anthony -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
