Which focus gear fits my DJI, Tilta, ARRI or PD Movie motor?

Any focus gear with module 0.8 teeth meshes with a DJI, Tilta Nucleus, ARRI cforce or PD Movie motor. You don't need a "DJI gear" or a "Tilta gear". You need the right diameter for your lens, enough width for the motor to grab, and module 0.8. The motor brand only becomes part of the equation when your gear has hard stops.

That's the whole answer. The rest of this article is the why, and the two cases where people still get it wrong.

Why the brand doesn't matter for a normal gear

Gear teeth come in sizes, and the size is called the module. Two gears mesh when they have the same module. That's it. Almost the entire film world settled on module 0.8: follow focus wheels, wireless focus motors, gimbal focus motors, the teeth on cine lenses.

So a module 0.8 ring on your lens doesn't care what drives it. Your DJI motor today, a Tilta next week, a manual follow focus on the next job. Same ring.

What does change between motors is the small gear on the motor itself: how many teeth it has and how wide it is. For a plain focus gear, that's irrelevant. The motor's gear rolls around your ring, whatever its size. If you want the maths behind the teeth: module 0.8, explained without the engineering degree.

One exception you might meet: older gear specified as 32 pitch. That works out to about 0.794 module. Close enough to mesh in most cases, not identical. Turn it a full rotation by hand and listen before you trust it.

What you actually need to check

1. Diameter. Measure the lens's focus barrel where the gear will sit. Not the filter thread. Here's how.
2. Width. The ring should be at least as wide as the motor's gear, so the motor meshes across its full width. Wider is fine, as long as it doesn't bury the aperture ring or a switch.
3. Clearance. Mount the motor and turn the ring through the whole focus range. Does the motor bracket, the rods or the camera body get in the way anywhere? A few millimetres of outside diameter can decide that. If you swap lenses a lot, give every ring the same outside diameter, so the motor never has to move.

That's it for a normal gear. The free focus ring generator does diameter and width for any lens from 40 to 110 mm. No account, and you don't pick a motor at all, because it doesn't matter.

When the motor does matter: hard stops

Hard stops are two physical stops on the gear at the ends of your focus travel. They help motors calibrate on lenses that turn forever, which is a lot of photo lenses.

For those, the motor's gear is part of the geometry. The stop has to hit the motor at exactly the right point, and where that is depends on the size of the motor's gear. A few teeth of difference move the stop to the wrong place. I learned that with my own first hard-stop rings, which knew exactly one motor: mine. The whole story is here, and when stops are worth it at all: hard stops for focus motors.

So for hard stops, you tell the generator which motor you use, or the tooth count and width of its gear.

Motor gear tooth counts

These are the presets in the hard-stop generator. I'm honest about where each number comes from, because a wrong tooth count gives you a wrong stop:

• DJI Focus Pro: 30 teeth (tested on my own rig)
• DJI RS Focus Motor (2022): 30 teeth (counted, not fully certain)
• ARRI cforce mini: 40 teeth (from my motor list)
• Tilta Nucleus M / MII: 39 teeth (manufacturer spec, not tested by me)
• Tilta Nucleus Nano: 26 teeth (manufacturer spec, not tested by me)
• Tilta Nucleus NII: 30 teeth (counted, not fully certain)
• PD Movie Motor Mini (PD-BTMP-N): 33 teeth (from my motor list)
• PD Movie Motor Smart (PD-BTMP-SPM), Pro (PD-RP), Air (PD-MX): 40 teeth (from my motor list)
• PD Movie Motor Smart / Smart Mini (PD-BTMP-SP / -S), Live Air 3: 33 or 40 teeth (sources disagree)

If your motor isn't on the list, or it's marked uncertain: count the teeth on its gear and measure the width. It takes a minute and you get the exact number for your motor instead of my best guess. The generator takes a custom tooth count and width.

The short version

• Normal focus gear: module 0.8, measured diameter, enough width. Any motor brand works.
• Hard-stop gear: also give it your motor, or its gear's tooth count and width.
• Not sure? Print the free slip-on ring first. It's the cheapest way to check your measurement on your lens and your motor.