Originally posted by Alex645 Shooting any lens wide open will not give you optimum sharpness across the field, but I do think it's decentered. Try shooting a flat brick wall at f/2.8. Try the same in the day of a city scape at f/2.8. Of course things will improve at f/5.6 or f/8....
This is the first time I think I'm quoting myself, but I think this is further evidence that it is decentered. Corners and edges are typically going to be soft wide open on wide angle lenses, but the fact that it is only happening in one corner indicates decentering. IMO you have these options:
a) Live with it and only shoot wide open when you don't need the entire field sharp.
b) Avoid shooting wide open and only shoot at f/5 or f/5.6 or greater if you care about the entire field being sharp.
c) Send the lens in to have the decentering fixed.
d) Sell the lens.
e) Always compose your shots with the issue in mind and crop it out in post.