Wormdrive Macro Rail Fine Focus Focusing Arca / RRS Lever Clamp Compatible 100% Metal 100% Metal Construction No Rubber or Plastic Parts. Precision focusing through a 55mm range. Index Scale on both sides 45mm diameter metal thumbwheel with 1/4"-20 Screw. Arca/ RRS compatible 55mm dovetail plate w 1/4"-20 threaded socket.