Fotasy 42mm M42 to M42 lens Focusing Helicoid Adapter 17mm - 31mm (M) This focusing Helicoid tube is made with standard M42 mount and is made with light-weight alloy. Both ends are 42mm. This Helicoid tube is useful for those lenses without focus ring. It could be used as Macro barrel, and offers fine focus control for macro photography. The length of this Helicoid tube 17mm (min) to 31mm (max), it offers a total of 14mm movement distance for lens focusing. User can use it with M42 screw mount lenses, such as Carl Zeiss, and even enlarger lenses. Very useful for enlarge lens, some of them are Leica 39mm / M39 / L39 mount. You may need an additional 39mm-42mm adapter if you want to use 39mm enlarge lens with the Helicoid tube.