A reversing ring reverses an interchangeable lens on the camera body. One side has a camera body mount, the other side has a diameter the same size as the lens’ filter thread you intend to reverse. A “Nikon AI to 55mm reversing ring” for example would allow a Nikon lens with a 55mm thread to be reversed on a Nikon body with an AI mount.
Reversing a lens makes it perform like a macro lens at a fraction of the cost.
To show the difference I mounted an Olympus 50mm Zuiko on an Olympus Micro 4/3 camera.
I then attached a reverse ring to the front of the lens
and turned it around to fit to the camera backwards.
I took a photo of breakfast cereal inside a plastic container with the lens in both the forward position
and the reversed position.
The lens was set to its closest setting of 0.45m for both photos. As you can see reversing the lens with a lens reverse ring allows much closer photographs to be taken.
I then took a series of photos with the lens reversed to show the type of photos you can take. Most of these are with the lens at maximum aperture so it gives a more creative effect with minimal sharp zone and lots of nice blurry backgrounds, like what you would achieve using a Lens baby .
As you can see reversing a lens is a creative way to add options to your camera kit for low cost.
We have a number of reversing rings for sale at photographyattic.com check out this tag page. Lens reversing ring