K-H, there was a lot of talk on that thread about the lens being de-centered and bad copies etc. I must be terribly lucky in this regard because I think my copy is pretty good.
I got it just before my trip to China earlier this month, and tested it out there. It is really quite a handy lens, surprisingly light and much easier to handle than the 24-70 GM (which is terrific in its own way). Of course it is f4 but then you can't beat the laws of physics I suppose.
I remember using a Nikon 12-24 with adapters on my Canon/Sony cameras and it was a nightmare, not being able to set the aperture at all. It was also a very bulky lens and although the IQ was very good, I feel IMHO that the Sony is equally good if not better.
Here are two images taken in the Forbidden City, both quick snapshots, as I was being pushed around by the crowds. They are completely unsharpened, no clarity either. All I did was adjust the exposure and minimal rotational correction in the first one. I did not even do that for the second image. When I took this it was raining, the skies were very dull and there was a crowd pushing me, despite all this I think it came out pretty good.
What is mind-blowing to me in the second image (also in the first) is that there is absolutely NO chromatic aberration, I deliberately did not correct the rotational error in this image to point to the tree in the upper left. This is where almost every other lens I have used will show some CA, given a white/grey sky behind the tree and branches. Also, note, the aperture is wide-open, f4 in both images.
I think Sony has knocked it out of the park with this one. Wide open, note that these are completely unsharpened images, see the focus is perfect edge to edge.
I just love it.
Sorry for the large image size, these are full frame full size images from the A7R2, wanted to show the details of the lens. Hmm....... even though I linked to the full size image, it shows up low-res here! Cannot see the CA-free corners unfortunately.