What are reversing rings used for?
Lenses that were not specifically designed for macro work, such as the Nikon 50mm f/1.8 “standard” lens, can often give better macro results when they are reversed (i.e., the back of the lens facing the subject). This is because general purpose lenses are not designed to work well at the large extensions (long distance between the sensor and the back of the lens) that are needed for close focusing; their optical formula has been optimised around the assumption that the subject-to-lens distance is always larger than the lens-to-image distance. If you want to focus so close that this relationship reverses, it is better to reverse the lens, and keep the lens working closer to its design assumptions. A reversing ring allows this to be done; it has a bayonet ring on one side, and in the case of a Nikon reversing ring, a 52mm screw thread on the other side. You attach the ring to the front of the lens, like a filter, and then fit the bayonet into the camera body, either directly or via exten
