This lens has a very convenient focal length range for digital SLRs. Its design is based on the very popular Tamron 28-300mm lens for film cameras (and full-frame digital cameras). But this convenience comes at a significant optical sacrifice - this lens is softer, and has more purple fringing and chromatic aberration than most lenses in its range, including the Sigma 18-200.
I'm surprised how much worse the Tamron's optics are compared to the Sigma 18-200, and I ultimately went with the Sigma, despite its front-focusing problem at 200mm (which I was able to avoid using a parfocal focusing trick - focus at 135mm, then zoom in to 200mm to take the shot).
Unfortunately, Epinions does not allow photos embedded into the reviews. Hence, I've posted a photo-enhanced version of this review, along with comparisons to the Sigma 18-200 lens, here:
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
Optical shortcomings:
The Tamron has so many optical shortcomings that I would recommend most people avoid it. If you really need the convenience of the 18-200mm range, then try the Sigma lens first, and see if you can coax it into focusing. If you can't, then the Tamron is a reasonable alternative if you don't make large prints, and if you don't mind a little Photoshop now and then to sharpen the images and remove chromatic aberrations. Because I often make large prints (bigger than 8.5"x11"), I found the Tamron's chromatic aberration, purple fringing, and softness to be annoying, and not worth the cost of this lens.
Here are the major flaws in more detail:
1. Red-green chromatic aberrations (CA). In 100% crops, these are much worse than the Sigma 18-200, especially in the corners at 18mm and 200mm. Fortunately, at normal print sizes, i.e. up to 8.5"x11", the CA is not as obvious, but can still be noticeable.
The best tool for CA correction I've found is the "CAfree" Photoshop plugin by Thomas Fiddaman, available here:
http://www.sd3.info/pf828/CAfree/CAfree0-1.html
I've posted 100% crops of my own results after correcting CA:
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
As you can see, I can almost eliminate CA. Many people correct CA with Adobe Camera Raw, but that only works on raw files, whereas CAfree works on JPEGs, or any other format.
2. Purple fringing. Purple fringing is much worse than the Sigma, although both have it to some degree. Purple fringing is worst at 200mm wide open, where it can be downright horrendous. Purple fringing can be mostly removed with another free Photoshop plugin by Thomas Fiddaman:
http://www.sd3.info/pf828/PFree/PFree0-1.html
Again, 100% crops of my results are on the geocities page:
http://www.geocities.com/tom_jhou/sigma_tamron_18_200.html
3. Softness. The Tamron is sharp in the center, even wide open, but degrades quickly away from the center, and the softness is most noticeable at 200mm. I frequently make 8.5x11" prints, and details are noticeably lost at 200mm, though photos generally look OK at shorter focal lengths or smaller print sizes. When compared to the Sigma 18-200, the Tamron is optically softer at all focal lengths. If you regularly make large prints, the Tamron is not a good lens choice.
4. Lens barrels rotate in the opposite direction of all my other Canon and Sigma lenses. This is quite confusing at first.
5. The Tamron has a plastic mount, which seems more likely to break than the Sigma's metal mount. The Tamron's internal gearing has a rubber drive belt, which is also more prone to failure than the Sigma's plastic gears. The Tamron's warranty is much longer - 6 years versus 1 year for the Sigma, so that should be some comfort.
Ultimately, I do not use my Tamron 18-200 enough to justify its price, and so I sold it. While the Tamron does focus well, and is convenient, I preferred my Sigma 18-200 in spite of its focus issues, as I was able to compensate using parfocal technique.
My other lens reviews
You can see all my reviews below. I'm particularly interested in lenses that combine telephoto reach with convenience.
Canon 100-400 - fast autofocus, good optics make very good wildlife lens. But loses some sharpness at 400mm, and becomes very 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, but there are many cheaper alternatives available now.
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 mode. But the camera doesn't do much in-camera noise-reduction, so you should get NeatImage or Noise Ninja along with this camera.
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
