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From: tab@ihuxl.UUCP
Newsgroups: net.rec.photo
Subject: Re: Reversing Rings
Message-ID: <393@ihuxl.UUCP>
Date: Wed, 15-Jun-83 09:26:47 EDT
Article-I.D.: ihuxl.393
Posted: Wed Jun 15 09:26:47 1983
Date-Received: Thu, 16-Jun-83 16:48:26 EDT
Organization: BTL Naperville, Il.
Lines: 39


      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.