I'm afraid that reverse-engineered lenses won't work well with the new Dual Pixel CMOS AF II system. I have the same problem with the R6mk2. It's a Sigma 70-200 Sport lens. It works flawlessly with the Canon R6, but unfortunately not with the R6mk2... This is what I've managed to determine, but it didn't help:
1. Update the lens firmware (Critical) Older Sigma Art lens firmware exhibits communication errors with the advanced AF system in the Canon R6 Mark II (e.g., focus drift, random misses). Solution: You need to update the lens firmware to the latest version using the Sigma USB Dock and SIGMA Optimization Pro. This update adapts the lens algorithms to the tracking and subject detection algorithms in mirrorless cameras.
2. Disable aberration correction in the camera. Processors in newer Canon cameras (including the R6 MK2) attempt to automatically correct optical aberrations in system lenses. In the case of third-party lenses (Sigma, Tamron), these features often cause software errors and autofocus "stupidity." Solution: Go to the camera menu (red shooting tab) > Lens aberration correction > Disable all options (Peripheral Illumination Correction, Distortion Correction, Digital Lens Optimizer).
3. Verify Object Detection Modes. The Canon R6 MK2 has an extremely sensitive eye/face/vehicle detection system. If you're aiming at a subject and the camera is set to the wrong detection category (e.g., looking for animals when photographing people), focus may drift to the background. Solution: For testing purposes, disable subject detection and switch the AF area to Spot AF or 1-point AF. Check whether the lens is hitting the target point when aiming statically.
4. Check the stabilization (OS) function. In Sigma Art lenses, a faulty optical stabilization (OS) can cause the lens to jerk when the shutter is released, resulting in a completely blurry photo, simulating a focus outage. Solution: Disable the stabilization on the lens barrel (OS OFF), place the camera on a stable surface, and take a photo with a shutter speed of, say, 1/500 s. If the photos are sharp, the problem is a faulty OS module in the lens.
5. Burst mode and performance drop (so-called "Focus Pulsing")EF lenses designed for SLR cameras (including this Sigma) in SERVO continuous shooting mode on mirrorless cameras may not keep up with the work of the aperture and AF motor, which causes every second or third photo in the series to be out of focus. Solution: If the problem occurs only in the series, reduce the burst speed from the electronic shutter to the mechanical one (\(12\text{ fps}\)) or switch the mode to ONE SHOT (single AF) for static frames to check for clear optical sharpness.
