Reviews and Problems with Sigma 17-70mm f2.8-4.5 DC Macro HSM
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Decentering issues in two copies, third isn't as sharp as previous
K. Yee "kenyee", Amazon
2 January 2009
Summary: Maybe I just have bad luck w/ lenses :-P The first two copies had decentering issues (left or right half of frame was fuzzy) but only at specific focal lengths and ranges...it took a few vacations and lots of shooting to see these issues. The third copy I got from Sigma seems to be a bit softer than the previous two...and they swapped my serial# label from my old lens onto this one (the first time, I got an entirely new lens with a new serial#).
Summary: I returned the first lens because it was back-focusing with my Pentax K100D. The second lens was ok, but strangely, softer than the first one in tests with a focusing chart (albeit no backfocusing). Both lenses appear softer than the Pentax kit 18-55mm, which is disappointing. I decided to keep the lens as it is the only 17-70mm that works with K100D body, is not absolutely terrible, and is not very expensive. If you buy this lens, check for backfocusing problems.