Hi,
I don't think Ken Rockwell shoots test charts.
Another point is that I don't agree folks showing corner crops on images focused at center are doing us no service at all. Many of the subjects we are shooting are flat. Architecture is often flat. A landscape shot at infinity is flat.
It is quite true that many subjects are three dimensional, so it is essentially impossible to have same focus across the image.
But, field curvature is not the only problem in corners. You can have astigmatism and chromatic aberrations, too. Than of course coma can be an issue.
I guess that reference is made to this image (Sony on top and Canon at the bottom, both linked from the Ken Rockwell site).
Full image:
I have gotten my 16-35/4 on past Friday. So long I have seen it performs decently across the field, but weather has been bad so I have just a few handheld shots outdoors.
Best regards
Erik
People who show us close crops of the corners from pictures that were focused in the center are doing us no service at all.
Ken Rockwell's pictures show us that EITHER the adapter is soft at the edges OR that it's making the field less flat, OR some combination of the two. If you don't take pictures of test charts pasted to walls, it's simply not an interesting test at all. I can't blame him, because this is what everyone does, because they're not actually interested in giving away, or discovering, information, they're interested in click-farming.