Fotasy M52 52mm to M42 42mm lens Focusing Helicoid Adapter (17mm - 31mm) This focusing Helicoid tube is made with light-weight alloy. The 52mm side is female mount and 42mm side is male mount. 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.