OK, Wayne - that is not what you want to see...the bottom corners, the left one further away than the middle of the (quite well rendered) bottom edge, are worse than poor IMO.
Why? It is clearly visible on the full 358kb web image...and blends with the vignetting to put a dark sludgy zone in a prominent area of such images.
It is disruptive to the scene's effectiveness, and as a photographer you will always see it. And there is no fix for it. That this is f8 adds to the problem.
I agree with jffielde that is also extends an unwarranted distance into the frame. And both these corners are at the end of composition lines - the line of rocks and the cliff face, where the eye naturally travels.
There is something worse though - the MTF indicates that even at f4, 40 lpmm (fine details like the rocks and texture seen here) lines dive after around 18mm - the middle of the wide edges.
BUT, so too do the lower spatial frequencies of 10 lpmm and 20 lpmm, they all drop very suddenly and are almost at the same metrics by 20mm out. By comparison, the 21mm lenses (both CY and ZE) still have these lower spatial frequencies at very high levels right out the very corner at 21.5mm image height. It appears the extra stop down (f4 > f8) does not change this performance.
What this means is the corners suffer both from poor detail drawing (40 lpmm) AND from loss of large object contrast/drawing - which in this image is just as bad an effect IMO. It is partly disguised by the strongish vignetting.
If you revisit the RAW file, you should see this effect just beginning on the sedges/rocks at the far right edge, level with the top of the rock spires (18mm from image centre).
Other than that, a lovely image - clean, brilliant and sharp, very good colour/contrast handling. Thanks for posting.