CanadaMark Offline Upload & Sell: Off
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If you are 100% sure your camera and the sign were perfectly square facing each other (you can check with a level), there is probably something wrong with that lens. It should be extremely easy to confirm. Level the camera and shoot a brick wall or something similar with uniform pattern detail. Your center crop wide open should have the exact same blurry section toward the bottom - if it doesn't, it's probably not the lens' fault. The fact that it sharpens up at F16 where DOF would be greater suggests that it was not perfectly square to the camera, but again, easy to confirm.
Your dot tune experience is exactly what I was telling you earlier, it is next to impossible to perfectly AF fine tune a zoom which may have 20+ focal length and subject distance combinations. You will be fiddling with that until you're blue in the face. Unless you get extremely lucky, a single value isn't going to fix a problem lens at all combinations. At that point, I would be exchanging it and not wasting any more time with it. Your "bad copy" luck is certainly not the norm as it sounds like you've have quite a few.
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