This is a simple 3D printed mount and sketch to turn a traditional follow focus into an input device. The original purpose was, well, pulling focus (in a CG shot), but the combination of high resolution, stable output, and marking disc make it useful for other applications.
Follow Focus: http://www.amazon.com/gp/product/B005ERZVX0/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1 (Cheap, as FFs go, and nice because you can dial in some resistance.)
Set screws (Optional. The hard stops annoyed me so I replaced them with M3 set screws.): http://www.amazon.com/gp/product/B000BMA1ZQ/ref=oh_details_o02_s00_i00?ie=UTF8&psc=1
Teensy 2.0++: http://www.pjrc.com/store/teensypp.html (You could surely use a simpler–and cheaper–board. I just happened to have one around.)
Pinion (32T 32P. A true 0.8 gear would be better but this worked): http://www.servocity.com/html/32_pitch__6mm_bore__gearmotor_.html
Rotary Encoder: https://www.sparkfun.com/products/11102
Breadboard: http://www.adafruit.com/products/571 (again, what I had lying around)