Here we can see the adapter and extension tube combination fitted onto the front of the EOS 5D, awaiting a Nikon lens. On the Nikon M-2 extension tube, there's a white-gray plastic lens release tab visible,and to the left of that you can see the white index dot on the M-2 tube. Nikkor lenses are aligned with the white dot,and are turned counter-clockwise,toward the C in Canon, to bayonet them on.