3D printed follow focus gear for DSLR lenses.
Use this printable diameter tape to measure the diameter of your focus ring.
Print on A4, at 100% scale.
Use the bottom scale to confirm it has printed at the right size (comparing with a ruler or tape), and then measure the diameter from the top scale.
Ensure the tape is pulled very tight around the lens as to ensure proper measurement.
It is very easy to measure 0.5-1mm too large if the tape is not pulled tight enough, and then the gear will be too loose.
Lenses are typically 70mm to 90mm in diameter.
If you are measuring >200mm then you are likely using the bottom scale on the print out, measuring the circumference not the diameter.