Hejnar Photo 8 Inch Screw Adjusting Macro Rail with Arca-Swiss Clamp 50mm high-precision macro focusing rail with 100 TPI adjusting screw drive and indexable 4-position quick release clamp. 8-Inch Hejnar Photo Arca-Swiss screw adjusting macro focusing rail with 0.004 inch fine control lead screw and quick-positioning button.