This is a module 0.8 gear ring specifically fitted to the Canon EF 24-105mm 1:4 L IS USM lens. It is intended to be used with a follow focus.
- It is thin enough to fit under the reversed lens hood.
- It has 120 inner teeth fitting precisely into the focus ring of the lens. (No slack.)
- The inner gear is beveled on one side so it slides onto the focus ring easily.
- It slides onto the lens from the bottom (i.e. the camera side). Since the diameter of the lens is a bit smaller where the AF/MF and stabilizer toggles are, it slides over those nicely.
- Does not require a clamp or bracket and thus allows for a full 360 degrees of rotation.
If printed with an Ultimaker 2 it fits perfectly onto my lens. However, it took me several iterations until it did. With a .25mm smaller diameter it wouldn't slide onto the focus ring at all while with a .25mm larger diameter it would slide off the focus ring way too easily.
So be prepared for a few iterations of fine tuning when printing this on your own printer!