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

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