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Mark's advice is good.
There's a commonly held belief that you don't have to be careful with camera shake/focus with wide angles.
Not so. Whilst it's true to say that the effect is more pronounced the longer the focal length, you still have to watch yourself and continue to obey the rules; Stable platform, tripod, beanbag, or yourself with elbows tucked in, camera properly supported, feet apart, shutter pressed gently, not stabbed etc. etc.
If you're going to have another go tomorrow, try an experiment and arrange a bunch of stuff with good sharp edges on a line leading away from the camera. Select one item about halfway down (not more than 0.5M) to focus on @ f3.5 and check that the focus is not long or short. It is unusual for this to happen, but it's not unknown. It may be necessary to send it back to Sigma for collimation.
Amateur Photographer magazine published a little gadget to check your focus. If you try their website I'm pretty sure it'll still be there somewhere.
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