The discovery

The rubber focus rings on Fujifilm GF lenses mesh with MOD 0.8 gears, the standard pitch used by cine follow focus systems. A DJI Focus Pro motor can drive a GF lens directly, with no gear ring or adapter in between.

With the GFX100 II set to linear focus and the DJI endpoints set by hand, I can pull focus from the DJI handle instead of reaching for the lens ring during a video take. It works better than I expected.

The setup

What I used:

How to set it up:

  1. In the camera, set Focus Ring Operation to Linear. In the default nonlinear mode, focus moves by different amounts depending on how fast the ring turns, so the handle's positions would never line up.
  2. Mount the motor so its gear sits directly on the lens's rubber focus ring.
  3. Set the endpoints manually on the DJI system. Focus-by-wire rings spin freely with no hard stops, so there are no physical limits for the motor to find.
  4. Turn down the speed and drag on the DJI handle, separately for each lens (more on that below).
  5. To set the endpoints:
    1. Turn the lens's focus ring by hand to its minimum focus distance, then set that as the first endpoint on the DJI.
    2. Turn the focus ring to infinity, taking care not to overshoot it. Use the focus distance scale on the GFX to check.
    3. Set that as the second endpoint on the DJI.

What I found

The lens and the DJI stay in sync, even through power cycles. I turned the GFX100 II off and on, let it sleep, and pulled and replaced the battery. Each time the lens came back at exactly the focus position I left it at, and the DJI handle still matched it.

I saw no slippage on the rubber and no endpoint drift. Both lenses I tested worked:

  • GF 45-100mm: a much longer, linear focus throw
  • GF 20-35mm: a much shorter throw than the 45-100

The one catch is tuning. I had to turn the speed and drag on the DJI controller down by a lot, and the right values depend on the lens. Because the focus throw differs so much, the same handle settings feel completely different from one lens to the next, so plan on tuning each lens separately.

Bonus: a quasi power zoom

The same trick works on the zoom ring. Unlike the focus-by-wire ring, the zoom ring is mechanical, so it has real hard stops at each end. Put a motor on it and any zoom lens becomes a quasi power zoom, with the lens's own stops as natural endpoints.

Why it works

Two things line up. The ribs on the GF focus rings match MOD 0.8 pitch, so the motor gear meshes the way it would on a cine lens. Linear mode then makes the focus-by-wire ring predictable: the same amount of rotation always moves focus by the same amount.

In hindsight, the MOD 0.8 rubber is a nice bonus, not the main point. A simple lens gear would work just as well. The bigger discovery is that the DJI endpoints act as repeatable hard stops, because the lens holds its position over time and never resets.

For GFX shooters, that means the GF lenses you already own can work with a motorized follow focus, no gear rings needed. I've only tested the 20-35 and 45-100 so far, so if you try this with other GF lenses, I'd love to hear how it goes.