Reverse adapter ring allows a normal lens (prime or zoom) to be mounted in reverse on a camera. When reverse mounted, a lens can be used to do macro photography work. The shorter the focal length, the higher magnification level can be achieved. Best of all, when using a reverse adapter ring, it does not matter if brand of lens matches that of camera because as long as the filter thread on the lens matches the adapter's. For example, a vintage 50mm Pentax lens with 58mm filter thread can be mounted on a Canon or Nikon camera if a 58mm reverse adapter ring is used for that camera.
Reverse Adapter Ring
Camera Mount & Filter Size