Hard stops: great, until they're built for the wrong motor

I like hard stops. A lot. When a lens doesn't give your focus motor a clear mechanical end, two defined stops make calibration so much easier.

But a printed stop isn't magic. It's a very specific part for a very specific rig. Three things have to be right at the same time: the ring has to fit the lens, the motor has to mesh properly, and the stops have to sit where your focus travel actually begins and ends. Get one wrong and you have a nice-looking part that makes the setup worse.

The part I got wrong

My first hard-stop design was based on my motor. I knew that setup inside out, so the dimensions felt obvious. Then I looked at other motors, and a few teeth of difference in the pinion were enough to move the stops to the wrong place.

Lesson learned: "module 0.8" isn't the whole compatibility story. It describes the tooth system, not the motor in front of it.

What you need before you generate anything

The lens. Find the band where the ring will sit and measure it, or use a preset if your lens is listed.

The focus travel. How far does the ring turn from close focus to infinity? Check it twice. Some focus rings keep turning past the useful focus range, which is where stops can help. With focus-by-wire, check whether the same ring movement reliably gives you the same focus change. Physical stops alone won't make a speed-dependent electronic response repeatable.

The motor. Which motor and which gear are you actually using? The tooth count isn't trivia, it's part of the geometry.

The hard-stop generator takes a lens preset or your measured circumference plus focus travel, and either your motor from the list or the tooth count and width of your own motor gear. The stops get calculated for that combination. If your motor isn't on the list, count the teeth and measure the width. Don't just pick whatever looks closest. That's precisely the mistake I made.

Test it like camera gear, not like a desk toy

Once it's printed, turn the ring by hand on the lens. Does it reach the focus range you need without forcing anything? Does it hit the rods, the camera body or another control?

Then run the motor calibration, slowly, and watch it. If the motor slams into a stop, slips, or runs out of range early, stop and fix the setup. A green light on the controller doesn't beat what you can see and feel.

And if your lens and motor already calibrate cleanly without stops? Then you don't need them. Start with a standard focus gear. I'd rather you print the simple part that works than a clever one you don't need.