Not quite. At 28mm, my CZ appears to be a bit better than his and his 28-75 appears to be a bit better than mine. The 24-105 are quite similar. At 70mm, my 28-70 is noticeably worse then his (at the center as well, so maybe I should indeed send this lens back for service), and the CZ and 24-105 appear quite similar.
As a side note: this test confirms what I already showed in this blog, namely that the 24-105 is indeed quite a good lens at f/8-f/11 (except for lateral chromatic aberration) …or rather that it is a good lens between 28mm and 70mm. At 24mm, the results between this lens and the CZ are vastly in favor of the CZ (even if the CZ corner sharpness is not very good at 24mm, the 24-105 is much worse).
But what about the CZ corners at 28mm? Are they as poor as Stephan thinks?
Let us examine the picture at f/5.6 again:
Isn't the right side sharper than the left side of this picture?
Let us look at the FTM again (from Sony Japan).
The scale at the bottom of this chart shows the distance to the center of a 24mmx36mm frame. So:
-12mm is the top or bottom edge of the frame (24 divided by 2)
-18mm is the left or right-hand edge (36 divided by 2)
-21.6mm is the distance to anyone of the 4 corners.
So what the FTM shows is that the picture stays sharp up to the extreme corners.
Let us see a bit more of the corner at f/2.8. The following picture is 800x1000 pixels, from the bottom left corner of the full picture. Here it is quite clear that the right-hand part of the picture is much sharper than the extreme corner. The top right corner is at 14mm from the center. I added figures to indicate the distance from the center in mm, so that you can compare the figure with the chart (but note that the chart is taken at 24mm and f/2.8 and this picture is at 28mm and f/5.6 so that the unsharp zone should be a bit smaller and further away from the center).
Note: that image is clickable and will open in its own window or tab if you want to examine it at the pixel level.
If we consider the full picture, the four corners will lack sharpness. Next, I tried to visualize the unsharp zone at f/5.6. Of course, this is an approximation: there is no clear limit to an "unsharp zone", but since the transition between "pretty sharp" and "pretty fuzzy" is rather steep on the FTM, we can draw some fuzzy limit, take or give some pixels. In addition, I drew the two crops: the small one as from Stephan and the bigger one from above (but without the numbers).
We find out that Stephan's crops are exactly from that steep transition zone.
Furthermore, we should realize that if the lens is not perfectly centered, that "transition zone" will move a bit (actually, the four corners will move and they won't be symmetrical). Could it be that Stephan's lens was not very well centered and that the crop ended in the worst corner? Only Stephan can tell, since he has access to the original pictures.
So what is the conclusion for us photographers?
Does the CZ have unsharp corners? Undoubtedly yes, even Sony acknowledges it.
Are the 28-75 or 24-105 better, then? Not really. I would say that the differences at f/8-f/11 are unlikely to be noticed on prints in practical use. At f/2.8, the CZ has better center sharpness than the Tamron (but not by much).
Does that mean that the price and weight of the CZ 24-70 are justified? Honestly, that is for you to decide. The CZ is better, but it is not perfect (which lens is?). However, in 90% of the pictures, you are not likely to see the difference in sharpness with the 28-75 unless you print huge enlargements or pixel peep. Then, the CZ is better built, has 24mm and SSM and the colors match better with the other CZ glass (16-35, 85 and 135mm). That and my Tamron does not work very well at 70mm, while I think it used to… YMMV, of course.
What if you want sharp corners at 28mm, then? Stephan suggests the Minolta 17-35mm f/2.8-4. The CZ 16-35mm f/2.8 is very good as well. What about 70mm? Stephan suggests the Minolta 28-135mm f/4-4.5. I don't own that lens, but the Sony 70-200 f/2.8 G is very sharp as well (see my test in November)...