A little more close detail of the Durus follow focus and the Proteus "dogbone" mounts on the 7D camera below and the 5D Mark 2 above. The Durus ff is ROBUST, but very smooth - allowing for very fine manual focus of even this Canon telephoto lens (which are notorious for their coarse manual focus). The extra large gear ring mounted on the lens goes a long way towards allowing this fine manual focus control.