LEGO Follow Focus & Fine Zoom Adjuster
This LEGO rig allows fine control of zoom and focus on a Canon 60D with the 18-135mm kit lens. The LEGO piece you are not seeing is the chain link that the gears fit into.
The left set of 2 big gears moves the focus ring. The right set of gears moves the zoom ring. The gears have knobs off of either side of the rig that allow very fine adjustments. The bottom two gears in the picture are simply hand holds, to make it easier to turn. Same goes for the ring thing at the top of the photo.
With some other LEGO blocks that have battery packs and motors you can even drive these gears with power.
The zoom and focus are completely independent.
The rack focus is especially nice on the 18-135mm lens because of the very small range of movement for the focus ring.