The all-metal adapter is designed to fit the lens securely, yet can still be removed easily when not needed. Introduction: This tube useful for lens without focusing ring. And it can be use as macro extension tube. There is no electronic elements in the adapter. aperture and focusing must be manually control. Material: aluminum alloy Made of Anodized Aluminum in Black finish with high quality. The length of this Helicoid tube is 17mm (min) to 31mm (max),it offers a total of 14mm movement distance for the lens focusing. You can use M42 screw mount lenses, such as Carl Zeiss or Pentax 42mm screw mount lenses, and even enlarger lenses. This Helicoid tube is useful for those lenses without focus ring, Tube offers fine focus control for macro photography. This focusing helicoid tube is made with 42mm front mount and standard 42mm rear mount and is made with light-weight alloy. It could be used as Macro Barrel. Automatic diaphragm, auto-focusing will not operate correctly while using this adapter. Manual focusing with A/M mode metering is ok with the adapter. Compatible With: M42 Lens & Camera Package contents: 5 x M42 to M42 Mount Lens Adjustable Focusing Helicoid Macro Tube Adapter - 17mm to 31mm