This mount adapter is compatible with Canon EOS cameras to attach T2/T mount lens. Diameter measures 6.5cm, inner hole diameter is 4.2cm, and bayonet diameter is 5.1cm. There is no electrical connection on the ring so diaphragm control of the lens is set manually. With a mini hex key to help tighten or loosen the screw. Easy installation, can be replaced directly without adjustment, great compatibility and high reliability.