19. december 2018
Initial testing was done under less than ideal conditions, but I can’t change the weather. I used a target, a steel mast approx. 680 meters or 740 Yards away so infinity focus should be reached by far.
Then I centered the camera on the mast and took a shot for each aperture, and then turned each of the 4 corners to the same target by turning and tilting the head on my tripod.
I know it's better to have a long distance so I can avoid tilting the camera and the optical axis, but that was what I could manage today, and I think this way to do it is OK.
(Please tell me otherwise in the comments if that is the case)
Unfortunately, the weather was a bit hazy and very windy, so I think a new test will be needed when the sky is more clear.
Anyway, let's start at f/1.8 with the center, and go to the bottom left corner and with the clock around till we reach the bottom right corner. Let me begin by saying that I am impressed! The corners are sharp and quite comparable in sharpness. Maybe the lower right corner is a tiny bit sharper but that is negligible. I had some varying sharpness because of shake, or haze, and definetly because of the strong wind, in some of the shots and choose the best ones but I am still in doubt about a couple of photos.
But there is certainly no serious centering problem at infinity and at f/1.8
The Photos are done with 5 sec. self timer, and focus on the center. There is NO sharpening applied to the photos, and no lens Profile either in camera or Lightroom. All files are RAW converted in Lightroom and exported as 100% quality Jpeg from LR. The crops are 100% crops from the very corner of the frame, so a very demanding test. See comments under each photo.
As a note I would never shoot a landscape at f/1.8.