Nikon lens inverter ring. The purpose of this ring is to connect the lens to the DSLR camera on the thread side (the outside of the lens). The metal ring has, on the one hand, a male lens mount to be attached to the bayonet of the DSLR camera. On the other hand, it has the same male thread that a normal filter would have. Therefore, there are different ring sizes depending on the thread diameter of the camera lens.