Focus rings, explained: what they are and how to pick the right one

Two things called a focus ring

Every lens with manual focus has one: the band you twist to pull focus. On a cine lens, that band already has gear teeth around it. On a photo lens it usually doesn't. It's a rubber grip made for fingers, and fingers are very forgiving.

Motors aren't. The moment you add a follow focus, a wireless focus motor or the focus motor on your gimbal, a small gear on the motor has to drive the lens. That gear needs teeth to bite into. So you put a toothed ring around the lens's own focus ring.

Depending on who you ask, that part is called a focus ring, a focus gear, a follow focus gear or a lens gear. Same thing. I named a whole website after it, so I'm going with focus ring.

What you need one for

• A follow focus. The classic wheel on the rods, turned by you or a focus puller.
• A wireless focus motor. DJI, Tilta Nucleus, ARRI cforce mini, PD Movie and friends.
• A gimbal focus motor. Same idea, smaller rig, and a lot less patience for a gear that wobbles.

In all three cases, the gear is the only connection between the motor and your lens. If it slips, rubs or sits crooked, the motor does its job perfectly and your focus still goes wrong. Finding that out on the desk is cheap. Finding it out in the middle of a take isn't.

1. Module 0.8: the number you check, not choose

Gear teeth come in sizes, and the size is called the module. Almost every follow focus and focus motor in film uses module 0.8. That's the good news: you rarely have to pick anything here. You just make sure your ring has module 0.8 teeth.

Some older gear is specified in imperial "32 pitch", which works out to about 0.794 module. Close enough to mesh in most cases, not identical. If you mix the two, turn the gear a full rotation and listen.

The tooth count then follows from the diameter. Because a gear can only have whole teeth, its outside diameter moves in steps of 0.8 mm. If you want the maths without the engineering degree, I wrote it up here: module 0.8 focus gears explained.

2. Measure the diameter where the gear will sit

This is where most wrong focus rings come from. The number on the front of your lens, 67, 77, 82, is the filter thread. It tells you nothing about the barrel the gear sits on.

Find the band on the focus barrel where the ring can actually live, then measure the outside diameter right there. Calipers work. So does a strip of paper: wrap it once around, mark the overlap, measure the length and divide by π. Measure in a few spots, because real lenses are less round and less tidy than the product photos.

Step by step, with the mistakes I'd rather you skip: how to measure a lens for a focus ring. If your lens is already in the lens library, start from those numbers and check them anyway.

3. Pick a width that leaves room

Wider means more teeth for the motor to grab. It also means more chance of burying the aperture ring, a switch or the spot where your hand wants to go. The free generator does 10 to 40 mm. Measure the usable width of the band, check where the motor's gear lands, and choose the narrowest ring that still meshes with the full width of the motor gear.

4. Slip-on, clamp or hard stops

There are three kinds of focus ring on focusrings.at, and you probably need the simplest one.

• Slip-on ring: a slim gear that holds by friction, for lenses from 40 to 110 mm. Free, no account. Start here, even if you think you need something fancier, because it's also the cheapest way to check your measurement.
• Clamp ring: three flexure clamps and M4 set screws hold it on the lens. For when the grip is slippery, the lens tapers, or the ring just has to stay put no matter what.
• Hard-stop ring: two physical stops at the ends of your focus travel, for lenses that give a focus motor nothing to calibrate against. These are calculated for your lens and for your motor's gear, so they need your focus travel and your motor model or tooth count. I learned the motor part the hard way, and wrote about it. When stops are worth it and when they aren't: hard stops for focus motors.

5. Material: PLA, PETG or ASA

For a slip-on ring, PLA or PETG is all you need. For a clamp ring, I use ASA with 6–7 walls, because it's much stiffer than PETG and the clamps only work if the ring doesn't give. PLA also works for clamp rings.

One thing to keep in mind: PLA gets soft at lower temperatures than PETG or ASA. If your rig spends summer afternoons in a car, that matters more than the colour.

Settings, shrinkage and the test ring that saves you a reprint: how to 3D print a focus ring that actually fits.

6. 3D printed, metal or a rubber strap?

All three exist, and all three have a place.

Rubber strap gears wrap around almost any lens and you can have one on in a minute. They work in a pinch. But the strap end sticks out somewhere, and the ring doesn't always sit evenly on the grip.

Metal gears are stiff and they last. They come in standard sizes or as adjustable designs, so you either pick the nearest size or pay for one made for exactly your lens.

3D printed focus rings are made for your exact diameter, your width and, if you want, one outside diameter for your whole set. A reprint costs a few grams of filament. The catch: the result is only as accurate as your printer, and plastic is softer than aluminium. That's why you print a test ring first.

My take: if you own a printer and shoot your own lenses, print first. You get a ring that fits your lens instead of a ring that fits most lenses. If your gear lives on rental lenses that go through a new crew every week, metal is the grown-up choice.

The trick worth stealing: one outside diameter for every lens

If every lens in your kit gets a ring with the same outside diameter, your follow focus or motor stays in the same spot when you swap lenses. No sliding the bracket between setups. In the generator, choose Outside Ø instead of Auto and type the same number for every lens. More on adapting photo lenses for this kind of work: make a photo lens work like a cine lens.

The short version

1. Measure the focus barrel where the ring will sit, not the filter thread.
2. Print the free slip-on ring at that size as a test.
3. Fits and holds? Done. Slips? Go to a clamp ring. Motor can't find the ends? Look at hard stops.
4. Write down the numbers that worked. Every ring has them engraved inside anyway, but future you will still be grateful.

No printer? Printing the parts myself is paused at the moment. You can leave your email on the homepage and I'll let you know when batch orders open.