Originally posted by 08amczb I changed from the Tamron to the Pentax 16-50 (used). I had problems with the Tamrons build quality, and I had focusing problems even after the lens calibrated in the service. The Tamron reported 50mm at the 17 end sometimes, I've noticed it with my flash. Altough the IQ was superb.
I have been happy with the Pentax as well, but the price is in another league. With the current prices on the Sigma, I'm not sure I'd do it again. Also, having used Sony bodies with the Zeiss 16-70 F/4 for a year, I'm not sure that F/2.8 is worth the bulk. YMMV.