A reversing ring, reverses the lens on the camera body. You need to choose one that matches your body mount with the filter thread on the lens. All auto coupling is lost so program exposure and shutter priority modes are not possible, but most cameras will work okay in aperture priority or manual mode using stop down metering. Canon manual focus cameras need a small adapter placed on the rear of the lens to lock the aperture down, while screw thread lenses need the aperture's coupling pin holding down if there isn't a manual switch. Canon EF autofocus cameras can't stop the lens down mechanically. A reversing ring offers an image that's razor sharp in the centre, but a little softer at the edges compared with a dedicated macro lens.