To quote Photozone about the sharpness:
Quote:
In terms of resolution the Sigma shows a bit of a roller-coaster ride throughout the zoom range. At 18mm the results are surprisingly good - the center resolution leaves nothing to be desired and the border quality is very good even at large aperture settings. At 35mm the situation changes dramatically. The center performance is still exceptionally high but the border quality is absolutely dismal at f/4 and f/5.6. The field curvature is quite hefty at this setting. It is advisable to avoid anything faster than f/8 at this setting. (FWIW, the Sigma 18-200mm non-OS showed also a performance breakdown at 35mm). Interestingly the weakness is gone at 50mm where the lens is capable to produce a pretty good and even center to border quality. At 100mm the lens shows a downward trend again - the center quality is very good but the borders and specifically the extreme corners are softer. The very good peak performance is reached at f/11 here. Surprisingly the border quality can recover again at 200mm.
Then again, sharpness is overrated, and the lens seems to do quite well in other fields. The stabilisation works, vignetting is kept under control, CA is decent for this type of lens.
Looking at that, I'd say it will not be an upgrade in sharpness (and not really a downgrade either, but if you want that extra 50-200mm in one lens...go for it!
Otherwise, consider an additional lens like a 70-300 (stabilised).
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