How to prevent autofocus but retain tethered focus control?

Question

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?

Answer 1

Set 'Custom Function IV: Operation Others → C.Fn-7: Shutter/AE Lock Button' to option '1: AE lock/AF'. It's on page 221 of the EOS Rebel T3/1100D Instruction Manual.

Pressing the shutter button physically (and hopefully via the Darktable app - I've never used Darktable rather than Canon's own software to shoot tethered) will only initiate metering, but not AF. To use AF you'll need to press the 'AE Lock' button physically (or hopefully also virtually with the Darktable app) that you have remapped to be the 'AF-ON' button. It is the button on the back of the camera near the upper right marked with an *.