Originally posted by donaldslevine the Sigma 17-50
I would highly recommend hanging on to that one- I too have the Sigma 17-50mm f/2.8 EX DC HSM and it is a great lens, especially for the cost. Performance is excellent, it was tested as having virtually no field curvature, a rare achievement for such a lens, and I don't find mine to be all that large for this type of lens. The new Pentax DA* 16-50mm f/2.8 PLM is much larger and heavier and costs a whole lot more. This Sigma lens handles well for me with the largest of the supplied grips. It is well-designed, with its AF/MF switch conveniently located and not requiring simultaneous switching on the camera body. AF is quiet, and with my copy is fast and accurate.
I have other lenses I use more frequently, like my long-time DA 18-135mm, or my very compact and excellent DA 20-40mm Ltd, which seems like it is made with the KP in mind, though it came out ahead of the KP. But when you need a pro-style high-quality constant aperture f/2.8 zoom lens, nothing else will deliver the shutter speeds for low light and/or faster action shooting as a lens such as this one can. I love it in combo with my DA* 50-135mm f/2.8 lens for a large aperture zoom kit with wonderful imaging quality, and at a fraction of the size, weight, and cost compared to the FF rig of a DFA 24-70 f/2.8 and DFA 70-200mm f/2.8 on the K-1 or K-1 II.