You're probably measuring the wrong part of your lens

77, 82, 67. The number printed on the front of your lens is the filter thread. Very handy if you're buying a filter. Completely useless for a focus gear.

The gear doesn't sit at the front. It sits on the focus barrel. Sometimes on a nice rubber grip, sometimes on a narrow strip squeezed between the aperture ring and whatever else the lens designer felt like putting there. So before anyone touches a caliper: find out where the gear can actually live.

First find the spot

Put the lens on the camera the way you actually use it. Where can the motor reach? What happens when you turn focus from end to end? Does the barrel move? Is a hood, an aperture ring or a switch in the way?

A perfect diameter in the wrong place is still the wrong gear.

Then measure it

Once you've found a usable band, measure the outside diameter right there. Digital calipers make it easy. Just don't squeeze a soft rubber grip until you get a nice, precise, wrong number. Measure in a few spots around the ring. If the readings differ, write that down. Real lenses are less tidy than the product photos.

No calipers? Take a thin strip of paper that doesn't stretch, wrap it around the barrel, mark where it overlaps and measure the length. That's your circumference. Divide by π (about 3.1416) for the diameter. Keep the strip straight. A slightly diagonal wrap adds millimetres you never had.

Measure the usable width too. The motor needs enough teeth to grab, but the ring shouldn't bury the aperture ring next to it. A slightly narrower gear you can live with beats a beautiful wide one that blocks everything.

If you want a clamping version, check where the screw will end up once rods and motor are on. A screw you can't reach isn't a feature.

Print one. Test it. Then commit.

The free Focus Rings generator turns your measurements into a module 0.8 STL. Treat the first print as a test of your measurement, not as the final part.

Let it cool, put it on the real lens and turn focus through the whole range by hand. Check for slipping, wobble, rubbing and tight spots. Only then add the motor, and run it slowly.

Printer, material and the lens grip all change the fit. That's why I trust a test print more than any number on a screen, including the ones I calculated myself. If it needs adjusting, change the value in small steps and write down what worked. A second print costs next to nothing. A gear slipping on set costs a lot more.

If the standard ring fits but you want it to clamp onto the lens, move on to the custom generator, with measurements you now actually trust.