It's probably softer on the right, because it's shot at F5 and focused on the left (closest tree) or am I not understanding you?
Could you guys check this "problem?" I noticed it very first day you post it...thanks.
All shots in this preview seems sharper on the left side...
The background in this shot looks softer on the right than on the left by quite a lot, suggesting either slightly de-centred lens elements or maybe a little play in the lens mechanism.
Another sunny day in Seattle.