How to prevent autofocus but retain tethered focus control?

by Ruslan   Last Updated August 12, 2018 12:18 PM

I'm using Darktable (on Linux) with Canon 1100D in tethered shooting mode. I found out that from live view section I can control focus in big or small steps. This all works nicely, and I can see the changes in the live view display. But if I click capture image(s) button in the camera settings section, then, before capturing, the camera starts its auto-focus sequence, which destroys all my manual fine-tuning. The only focus-related modes I found are "One Shot", "AI Focus" and "AI Servo". No programmatic option to disable auto-focus.

What I tried:

  • Initial state: have focus control from PC. It requires AF mode of the lens, so this is basically useless since on actual capture I lose all these settings.
  • Switch the lens to MF mode. This disables auto-focus, but also disables the tethered focus control – no use.
  • Mechanically toggle the AF-MF switch: AF for tethered focus control, MF to capture. This sort of works, but mechanically disturbs the camera, affecting the direction of look as well as slightly changing the focus setting – bad usability.

I also thought of trying to save the live view image, but: 1. this won't give me raw data (live view is JPEG); 2. its shutter speed and ISO sensitivity are simulated, not real, so these settings would also be unusable.

As you can see, all the above options are flawed. So, how can I correctly disable auto-focus, but still retain focus control from PC? Is there maybe some hidden setting to disable auto-focus programmatically?

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