In this photo, the M-2 extension tube's white alignment dot is indexed with the black dot which is on the small flat spot on what Nikon enthusiasts used to call "the silver ring", which is something Nikon used to have on all of its lenses. The Nikkor lens,which is on the right, is ready to be turned towards the camera position,so that the index dot and the f/5.6 aperture marks will all come into alignment with the red dot that is seen close to the Canon logo on the 5D's pentaprism. Just a short,quick turn,and the lens will be fully bayonetted on and ready to shoot.