XILETU All Metal Wormdrive Macro Focusing Focus Rail Slider/Close-up Shooting Clamp Plate: 115mm Adjustment with 1/4 inch Screw Head for DSLR Cameras, Tripod Ball Head, Lever Clamp Arca/RRS Compatible. Introduction SOLID CONSTRUCTION: Made of metal, durable and strong; metal construction for greater durability, ensures years of corrosion and abrasion resistant operation. DESIGN: Macro focus rail sliding plate focusing with a 115mm adjustment range, with two control knobs, can be convenient for manual regulation, helping to achieve precise distance between camera and subject. FLEXIBILITY: The rail is driven by worm gear endless knob controlled and index scales which are printed on both sides of the rail in millimeters for consistency. COMPATIBILITY: 48mm diameter metal thumbwheel with 1/4 inch screw , cameras can be mounted on the top of the rail, the base is 38mm wide Arca type plate, with 3/8 inch bottom thread, you can attach it to your tripod, ball head. Standard: Compatible with Arca-Swiss RRS standard.