This is an extension tube for M42 mount lenses. L=28.5mm (measured with calipers) It comes with a case. The original case might be a bit rare. It allows for close-up photography with M42 mount lenses. This extension tube is convenient when using M42 lenses with a digital camera and a mount adapter. Since the L is large, it will be easier to use with lenses with a long focal length. Asahi Optical is PENTAX, and the logo on the case is the old type. Please consider it.