Camera lenses are designed to perform optimally within a specific range of focusing distances. General-purpose camera lenses usually perform at their best when focused between infinity and about 0.5 m. Macro lenses, on the other hand, are designed to be sharpest at closer distances.
When using a general-purpose lens to take pictures at high magnification (usually mounted on extension rings or bellows), the lens is often closer to the subject than to the focal plane of the camera. In these conditions, reversing the lens (i.e., mounting it with its back element facing toward the subject) makes it work in conditions that more closely resemble those expected by its designers. Consequently, optical aberrations will be lesser in this way. Macro lenses designed to provide reproduction rates up to 1:1 should also be reversed when used at substantially higher magnifications. On the other hand, specialist macro lenses designed for high-magnification work should never be reversed.
Mounting a lens in a reversed position requires a special coupling ring, like the BR2A. You may need also a reducing or enlarging filter adapter ring if the lens you intend to use has a filter attachment of different diameter than the reversing ring.
Enlarger lenses may also need to be reversed in order to perform optimally. This is possible if the lens has a filter thread (or a second lens attachment thread, like some EL-Nikkor lenses) at its front.
Some bellows for macro photography allow the lens-carrying standard to be reversed. In this case, the front of the lens (now facing toward the camera body) must be coupled to the bellows with a filter adapter of appropriate diameter. The above figures show the process of mounting a reversed lens on Olympus OM bellows.
One of the advantages of reversing the lens standard of bellows is that the controls of the bellows can be used to close the lens diaphragm. In many camera lenses, the diaphragm remains fully open when the lens is not connected to a camera, and therefore a reversed lens cannot be used properly. In these cases (which include Olympus OM lenses), if bellows are not used, one must resort to mounting a special adapter ring, containing cams to close the lens diaphragm, to the lens bayonet. Nikon lenses, on the other hand, close the diaphragm when not mounted on a camera. In this case, when not using bellows with a reversed front standard, one must open the diaphragm manually (by turning the aperture ring) before composing and focusing, and close it to the appropriate aperture before shooting. The same procedure must be followed when using enlarger lenses (whether reversed or not) and special-purpose macro lenses like the Zeiss Luminars and Leitz Photars.
Lenses can be reversed also by using a special adapter ring. The above picture shows the Nikon BR2A reversing ring connecting a reversed Micro Nikkor 60 mm f/2.8 (via a Kood 62-52 mm step-down ring) to Nikon PB-6 bellows and a D200 camera. With this type of set-up, the diaphragm must be closed manually before shooting. The Nikon E2 extension ring facilitates this task. It has a button to open temporarily the diaphragm while focusing. Releasing the button closes the diaphragm to its preset aperture. This ring is also useful as a lens shade when mounted on a reversed lens.
This setup is different than using a reversed lens by itself. A reversed lens can be attached in front of a lens mounted normally by joining the filter rings of the two lenses. This has the purpose of reducing the focal length of the combined lenses, in order to allow focusing at closer distances.
Usually, this setup works if the reversed lens is of shorter focal length than the lens mounted normally. Otherwise, severe vignetting may result. A reversed lens of shorter focal length will provide a higher magnification. For high magnification work, it is desirable to mount a reversed wide-angle in front of a medium telephoto. A wide-angle with a retrofocus design (which is true of all modern wide-angles) provides the added advantage of an increased working distance between lens and subject. In spite of this, the working distance can be extremely short, and some combinations of lenses don't work at all (sometimes, the subject would have to be placed inside the reversed lens). In addition, it is best to keep as short a distance as possible between the two lenses. In general, it is not possible to predict in advance which combination of lenses will work. The best way to find out is by experimenting.
In this setup, the diaphragm of the reversed lens is kept wide open, and aperture is controlled only with the diaphragm of the lens closer to the camera body. Closing the diaphragm of the reversed lens usually results in vignetting. This setup has the same issues with controlling the diaphragm of the reversed lens that concern the use of reversed lenses in general (see above).
Attaching two lenses front-to front in order to obtain the setup described above requires a special coupling ring. For experimenting, two lenses of similar diameters can be coupled together temporarily with duct tape, but an extended use calls for a coupling ring. This ring should be as thin as possible, in order not to increase the distance between the two lenses. Doing so would cause the working distance to decrease, and other potential problems to arise.