One item to note - if you are shooting close up at 200mm, the 18-200 will
not yield 200mm! All mega-zooms have this, the mechanics of such a long range makes it impossible to do cheaply. It will do 18-200 at infinity, but at 10 feet it's a close match to 140mm on my 50-200. Cameralabs website has a great story about it, look in their Nikon 18-200 review and there's a page that taught me what I know.
Aside from that, the DA(wr)50-200 that I have shows more vignetting but both have very good IQ on the k200d.
p.s. I owned a Sigma 18-200 in my Alpha days, more compact and a touch brighter than the Tamron (it stayed f/5.6 out to 180mm). I liked that lens a lot, but you'll note its price is higher than the Tamron. Also note that the 50-200 is a kit telephoto, so you're hearing conflicting reports because kit lenses generally have poor quality control. Some will get lemons, others solid gold; most of us have intermediate shades of yellow
.
Similar Threads 
