Decentering has nothing to do with the quality of the design and the material that goes into the lens. It's plain simply: They didn't assemble it right.
So a lens from Sigma (or Tamron btw.) can be very good if they just learn how to assemble their lenses with less junk getting off the line.
And I'm certainly not worried about corners being softer than the centre. But if you find copies of the same lens with clearly better corners than other copies you know that these guys are only sloppy in assembling their gear. And that is really infuriating
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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