Coupling rings are adapters with a male filter thread on each side. They allow you to connect two lenses by their filter mounts. The lens that is furthest from the camera is mounted in reverse and acts as a powerful and well-corrected close-up lens. A 50 mm lens is +20 dioptres, much stronger than normal close-up lenses.
The reversed lens should be used at maximum aperture. The two lenses should be as close together as possible, so it is best to remove any filters before coupling. Long and/or heavy lenses should not be reversed, because they would place too much strain on the filter mount.
If the lens that you are reversing incorporates floating elements for better performance at short distances (like some macro lenses and fast wide-angle lenses), then you should test it set for close focus and set for infinity focus; one position will probably give better results.
Because the reversed lens is attached by its filter mount, you are not restricted to using lenses designed for your camera system. You can even reverse enlarging lenses.
Some combinations of lenses cut off the corners of the image or even produce a circular image; this is caused by the diaphragm of the reversed lens. This effect is greatest when the main lens has a short focal length and the reversed lens has a small maximum aperture. You may be able to avoid this effect by focusing the main lens at its closest distance or by using an extension tube.
Reversing a lens exposes the rear element and various coupling mechanisms that are protected when the lens is mounted normally. Turning the focusing ring may extend the outer part of the lens to provide some protection for the rear element; this works with Olympus lenses. You can also provide them with some protection by adding a short extension tube to act as a makeshift lens hood; an old manual tube will do. You can also buy retrostep rings from SRB Film Service that allow a filter, lens hood or ring flash to be attached to the rear of a lens.
|Main lens||Reversed lens||Working distance||Subject size||Magnification|
|200 mm||24 mm||40 mm||4.5 × 3 mm||8:1|
|200 mm||50 mm||40 mm||9 × 6 mm||4:1|