Epinions doesn't allow embedded photos, which is a shame for camera/lens reviews where the photo is the final product. Hence, I've posted a photo-enhanced summary of this review on my Geocities site:
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
All the info below is also on the site above, so if you want to see actual photos (well, actually 100% crops of photos) please paste the above link into your browser. Thanks!
This lens is optically good for its wide range, and much better than the Tamron 18-200, which has more chromatic aberration, purple fringing, and corner softness. Unfortunately, I was unable to get a copy that focused properly at 200mm for distant objects. I tried three copies, on three different Canon DSLR bodies (two 300Ds and one 350D), with the same results, whereas my other lenses focused fine. Fortunately, this lens is parfocal, so I can focus at about 135mm, where the autofocus is still accurate, then zoom in to 200mm to shoot. Cumbersome, but it works, and yields photos that are surprisingly sharp at 200mm, coming close to the performance of the Canon "L" lenses I've used, at least in the center (but of course, the "L"s are much better all-around, with faster autofocus and better corner-to-corner performance).
Not all Sigma users have reported focus problems near 200mm, but at least one highly-regarded photography site, Photozone, confirms the presence of focus problems at 200mm, using that flaw to conclude that "there is no free lunch":
http://www.photozone.de/8Reviews/lenses/sigma_18200_3563/index.htm
The photozone site conjectures that the focus errors are due to the small (f/6.3) aperture at 200mm, but this can't be the whole story, because my Tamron 18-200 lens focuses perfectly at 200mm, yet is also f/6.3. You can read my thoughts on the Tamron here:
http://www.epinions.com/content_192627838596). I have also had a similar focus problem with Sigma's 18-125 lens, also for distant objects, and its aperture is a larger f/5.6.
I have also found one other site showing direct evidence of the Sigma's focus problem:
http://www.pbase.com/ingor/sigma_18200_test
Incidentally, Photozone does not find focus problems with the 18-125.
Summary of misfocus issue
All 3 copies of the Sigma that I tried front-focused at 200mm with distant objects (i.e. over 30 feet away). The lenses focused well for nearer objects, but in real-life situations, it is often very distant objects that I want to zoom in on. So the focus errors at 200mm were pretty significant. My camera focused fine with all my other lenses.
To show how bad the focus can get at 200mm, I've posted the a 100% center crop of a sample image. To see it, look at the first image posted here:
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
Many people say the Sigma gets soft at 200mm. When manually focused, I don't find this to be true, so I suspect the softness is actually due to misfocusing. When manually focused, my Sigma lens at 200mm is almost as sharp as my 100-400L in the center (though not at the edges, where the L is obviously better).
Sigma can't fix the focus problem
After returning two misfocusing 18-200 lenses, I sent my 3rd copy to Sigma for calibration. I included a detailed description of the problem, along with photos. It came back with a note denying any focus problem. Very frustrating. I also spoke on the phone to a Sigma representative, who basically admitted that Sigma doesn't understand Canon's complicated autofocus system very well, and said he isn't sure Sigma can make my lens work with my camera. Not a huge vote of confidence, but at least it was refreshingly honest.
On the bright side, this lens is still pretty useful, because it focuses fine up to about 135mm, and can be coaxed into working above that using parfocal technique. Nonetheless, if you buy this lens, make sure your dealer has a good return policy that will allow you to return this lens if it does not suit your needs.
Other minor flaws
This lens also has some optical flaws:
1. At 18mm, f/3.5, this lens gets soft in the corners. It's somewhat worse than the 18-55 kit lens, but much better when stopped down. I would not recommend it in low-light situations, but otherwise it's fine.
Incidentally, at 18mm and f/3.5, images from my copy are softer on the right edge than the left. This has also been reported by others, and also goes away when stopped down.
2. Slight chromatic aberration (CA) in edges and corners. Almost all consumer zooms have this problem, including the Canon 18-55 and 17-85 IS lenses. Interesingly, the 18-200 has much less purple fringing than either Canon lens, so overall the mega-zoom 18-200 has fewer aberrations than other lenses with much less range. The CA is only visible on large (8.5"x11" or larger) prints, and can be removed with the following Photoshop plugin:
http://www.sd3.info/pf828/CAfree/CAfree0-1.html
The above site also has a plugin for removing purple fringing.
3. Does not actually reach 200mm with nearby objects. When focused on objects at moderate distances (less than 30 feet away), this lens only reaches about 160mm at full zoom. Reportedly, this is due to the Sigma's internal-focus design. The shortcoming is less apparent with distant objects, but of course, that is where we have the misfocusing problem.
Build quality
Having owned the 28-300 lens, I expected this lens to be built similarly to that earlier lens. Surprisingly, it has a nicer feel than the older lens. The plastic has a matted soft finish that is pleasant to touch. The old 28-300 had a soft squishy barrel component (only exposed while zooming), while the new lens is sturdier. Incidentally, the Sigma lens has a metal mount, unlike the Tamron 18-200's plastic mount.
This lens is also slightly smaller than the 28-300, even though it uses the same size filters (62mm). I happen to like smaller, lighter gear, and the 18-200 goes nicely with the rebel XT, which is also astonishingly small.
Comparison with Sigma 28-300 macro
This lens has a very similar design as Sigma's earlier 28-300 macro lens, but it has less macro ability (only 1/4.4 versus 1/3 magnification). This was a minor disappointment, but only minor, when compared to the focus problems.
Alternatives
A lot of people considering this lens will also consider Sigma's 18-125 lens, and Tamron's 18-200mm lens. As I mentioned above, some people have reported focusing issues with the Sigma 18-125 also, though many have not. In my own experience, Sigma is uniquely frustrating with its combination of great optics, uniquely useful focal ranges, and inconsistent focusing. Tamron lenses trend in the opposite direction for me - tending to focus well, but to have more severe optical shortcomings.
Summary:
Ultimately, this lens wasn't what I was looking for. In sunny outdoor situations, where this lens performs best, I prefer to use a good point-and-shoot, due to its portability. I am most likely to use my DSLR in dim or challenging lighting where a point-and-shoot just won't work. In that case, I choose faster-aperture lenses like my 35mm f/2. Even if it focused perfectly, the Sigma 18-200 wouldn't serve my needs, and so I ultimately sold it.
Other lens and camera reviews
I have used and reviewed the following lenses and cameras:
Canon 100-400 - fast autofocus, good optics make very good wildlife lens. But loses some sharpness at 400mm, and becomes difficult to use with 1.4x teleconverter.
http://www.epinions.com/content_171720740484
Sigma 80-400 - good optics, autofocus too slow for wildlife:
http://www.epinions.com/content_170855927428
Tamron 200-500 - faster autofocus than Sigma 80-400, but still not fast enough for flying birds:
http://www.epinions.com/content_192416288388
Sigma 18-200 - good optics, versatile range, but my copies misfocus near 200mm (though I can coax it into working with parfocal technique).
http://www.epinions.com/content_185158569604
Tamron 18-200 - worse optics than Sigma, but it focuses accurately at all focal lengths on my camera.
http://www.epinions.com/content_192627838596
Canon EF-S 10-22 - Wonderful wide range. Slightly expensive, but good quality.
http://www.epinions.com/content_177984736900
Canon EF-S 17-85 IS - I liked this lens, but not enough to pay $600.
http://www.epinions.com/content_176145862276
Sigma 18-125. My first attempt to use a Sigma DC lens. Had front-focus problem at all focal lengths. Did not try a second copy as I switched to 18-200.
http://www.epinions.com/content_152606051972
Pro-optic extension tubes. Cheap in every sense of the word.
http://www.epinions.com/content_177304342148
Sigma 28-300. My first lens for my Canon 300D, purchased before Sigma released any DC lenses. It was pretty good considering its wide range, and focused quite accurately. But 28mm isn't very wide on a DSLR.
http://www.epinions.com/content_153463459460
Nikon 24-120 VR. The only Nikon lens I've used.
http://www.epinions.com/content_150456012420
Digital Camera reviews
I have also used a handful of small digicams, in my quest to find the perfect travel pocket camera. Here are my reviews:
Ricoh Caplio R3 - my favorite small camera, with its 7x optical zoom, 28mm wide angle, astonishingly small size, very good image color and detail, image stabilization, and voice recorder. Camera's image-processing is not as good as Canon models, but can be corrected in Photoshop (which is rarely actually needed).
http://www.epinions.com/content_217558847108
Sony DSC-S40 - very small, good movie mode, but unusually poor low-light performance:
http://www.epinions.com/content_180326272644
Canon S60 - very good image quality, but very slow operation:
http://www.epinions.com/content_184597712516
Panasonic LZ2 - small size with big zoom, but colors are somewhat unnatural, particularly the fluorescent green foliage:
http://www.epinions.com/content_182222098052
