coltoff@burdvax.UUCP (06/14/83)
I have some simple questions for those of you out there who use
reversing rings. I've put them on my short lenses ( i.e. 50mm &
28mm ) but want to try it with my 70-200. Will trying this with a
longer lens cause any thread damage to the lens or the reversing
ring or this a mechanically safe thing to do? How much magnifica-
tion can I expect with this type of setup? And at his point
should I just break down and buy the bellows?
Thanks in advance
Joel Coltoff
{presby,psuvax,sdcrdcf}!burdvax!coltofftab@ihuxl.UUCP (06/15/83)
With an ordinary lens (i.e., not optimized for 1:1 magnification),
use a reversing ring ONLY at magnifications greater than 1:1.
Using a reversing ring with a lens that
is not extended far enough from the film plane to reach 1:1
magnification will degrade the image. This degradation shows up
as color fringing near the edges of the image and inability to focus
on the center and edges of the picture simultaneously.
To use a 200mm lens at a magnification great enough to justify
use of a reversing ring you will need a bellows.
EXPLANATION:
Look a cross section of a camera lens and you will see that it
has front elements, back elements, and an aperture in the middle.
If the lens is optimized for 1:1 magnification (e.g., the lens in
a photocopier), then the front and back elements will look like mirror
images of each other; i.e., the lens is "symmetrical". Such a lens
performs best at 1:1 and worst at either infinity focus or very high
power magnification. A reversing ring would have no effect, since
the lens is symmetrical -- having no well-defined front or rear.
On the other hand, ordinary camera lenses are "unsymmetrical",
with front elements that do not look like the back elements.
Because they are unsymmetrical, they have a well-defined front and rear.
They are optimized for best performance at infinity focus, where
the tiny image on the film is very close to the back elements of the lens
and the large subject is very far from the front elements.
Notice that if we turn such a lens around backwards, it will have
its best performance at very high magnification where the tiny subject
is very close to the rear elements of the lens, and the enlarged image
is very far from the front elements (i.e., large bellows extension).
That is, the lens does not degrade the image as long as the front
elements are pointed at something (either subject or film) very
far from the lens and the back elements are pointed at something very
close. Note that such a lens will not perform well at 1:1 either
with or without a reversing ring.
In summary, for long distance and very low power magnification,
use an ordinary (unsymmetrical) lens. For magnifications around 1:1,
use a macro lens designed for the purpose (symmetrical or nearly so).
For magnifications much greater than 1:1, use the ordinary lens again,
but with a reversal ring and large bellows extension.