[skysurfer]

I made a shot of the Cygnus nebula region with the Canon EOS 6 with EF 85mm f/1.8 (set to f/2.5 to get rid of the worst vignetting) and an Astronomik UHC-S clipin filter, the latter due to moderate light pollution.
I focused using Live view via DSLR controller on my Android cellphone screen (much larger than the rear camera screen) and took a few test shots which I downloaded to my cellphone and zoomed in strongly and everything in sharpness (near the center) seemed OK, so I continued with the two minute frames.

But the left (south) is out of focus. And this is not a stacking error as the original frames (30 x 2minutes) have the same issue.

It is not the first time I got this error.

Here the images: the stacked (with Photoshop CC 2018) and postprocessed image and one of the original frames, downscaled to 1920x1280 in jpg.

It is not the sensor as most photos are just sharp to the edges, particularly taken with the 70-300L at full aperture f/5.6.