Lens Shootout: DA* 16-50/2.8 Vs Sigma 18-50/2.8 EX Vs Tamron SP 17-50/2.8

http://www.pentax-forum.net/viewtopic.php?f=11&t=25

(in Slovenia, Google Translated English Page Here)

I think the review article is self-explanatory and the posted pictures speak themselves. Just go measurbate and compare! Good luck!