Originally posted by csa Been reading the reviews on this one as well!
It is marvelous. Wonderful image quality and build. If there's concern about the old SDM (mine has been fine for 10 years!), then this lens can be converted easily to screw-driven with an increase in AF speed. Similar to a 70-200mm f/2.8 on a FF body, but MUCH more compact, and MUCH lighter. Less than 1/2 the weight. With its IF design, it does not change its relatively compact size at all during AF or during zooming. Nothing moves!
I also have the Sigma 17-50mm f/2.8 EX DC which makes a very good pairing with it. A very nice, fine-performing and well-made lens.
If you google the lens name, one site to check into is Imaging Resource, which in some cases presents a full test report, including a visual test shot of a still life scene, the same for all lenses. I tend to disregard expert opinions, even of the testers themselves, and just examine the visual results for myself. They provide both wide open and mid-aperture examples for you to select from. You can then pick a central area of the scene, which is the basket edge, and you will get a huge blowup of that one area- then you can move around to other areas from there. Check detail in the figures on both the Hellas and the Fiddler's Elbow bottles, as well as writing on the Samuel Smith bottle. For edges, which are not usually quite as good as the central area, just move over to the far brush and circular numerical scale.
Like virtually all zoom lenses, this Sigma lens it is not quite as good at its longest end, but in this case still quite good, and I found just a click to f/3.2 brings quality to an even better range, very good indeed, even in the excellent range. It has very little field curvature, which is surprising for a zoom lens, according to Optical Limits testing. Its main competitor, the Tamron, has not the build quality or quiet AF, users reported underexposure wide open, reporting it as actually being an f/3.2 lens to get proper exposure, and has a reportedly higher than average field curvature, according to Optical Limits.