Simply pull back focus towards the camera as Canon's EF 20/2.8 has sharpness field curvature.
It is important to remember about it when we shoot landscape photos and we want the foreground to be pefrectly in focus.
I used Canon EOS 1Ds mark III.
Aperture on all photos f/8.0
Here the focus is near infinity so we can see very soft corners as they are simply out of focus.
I pulled back focus towards camera to increase sharpness of the foreground and in the corners which are still soft.
I kept pulling the focus and now sharpness is O.K. I think to get even better result I should stop down to f/11 or try to focus even more precisely.
Now when I pulled too far only the corners are sharp!