Conclusion: Two questions, this test should give answers to, concern the cost-effectiveness of changing the kit lens 18-55 mm for Pentax 17-70 mm and a choice between the Pentax and the Sigma with the same focal length range. To tell you the truth non of these answers can be straight. When we replace the kit lens with the Pentax 17-70 mm we get a biger focal length range, a faster lens and a better autofocus. Is it worth the pain?
Cons: the manual focus ring moves too easy and is too small,, weak against the bright light performance,, too little frame corner resolution at 17 mm.