Got this lens, and started taking a few shots with it... alas, it seems soft in the corners. Perhaps a centering issue ?
At Paris CDG Airport (handheld):
The picture looks nice and sharp... excepted that top left corner.
Some tests I later made, using a tripod, at 12mm... (100% crops)
At f/4, center: nice and sharp, as expected from this lens
At f/4, left border: somewhat softer close to the border, but really good for a border
At f/4, top-left corner: VERY soft in the top left, the blur gets in very quickly and massively
The right side corners are also soft, although to a slightly lesser extent (the soft area is smaller)
At f/5.6, center: still very sharp, even somewhat sharper... as expected
At f/5.6, border: still good... but already somewhat worse than before at the very border ? Hard to tell...
At f/5.6, corner: still very blurry... and apparently even worse than at f/4
At f/8, center: no surprise, very sharp
At f/8, border: stopping down doesn't improve this area that much, it looks even worse the more it's stopped down
At f/8, corner: still at least as bad as before... if not even much worse
Of course, I won't exclude that my test isn't perfect - I was slightly misaligned with the wall, but at this focal length the depth of field is quite large... besides, I did multiple tries and moved the focusing ring, the results were the same. And as stopping down doesn't improve the blur, I'd guess it's not just due to being out-of-focus (which, anyway, wouldn't have made such a strong transition from sharpness to blur).
What are your thoughts about this ? I'm wondering if it's normal at 12mm and I just have unrealistic expectations, or I should send the lens back for servicing/exchange (2nd hand but still under warranty). Or could it be the camera itself ?
Looks like I'm condemned to shoot wide open, and not close more than 5.6 without some cropping...