Originally Posted by Braciola
What about the new Sigma 17-50mm F2.8 EX DC OS HSM ?
I'm a little confused on the Sigma's, they have a couple of versions in the 17-50 , 18-50 range.
Which particular model has the best IQ and will work with the Kx? .............
17-50mm F2.8 EX DC OS HSM
18-50mm F2.8 EX DC Macro
18-50mm F2.8 EX DC Macro HSM
The designation of OS
refers to the Optical Stabilization feature of the lens, which is useful for Canon and Nikon cameras that don't have sensor shift image stabilization in the bodies. Your Pentax K-x has stabilization in the body, so you don't need it in the lens too.
The designation of HSM
refers to the lens' HyperSonic Motor, which is useful for Nikon camera bodies that don't have autofocus motors of their own. For some of those lenses, at least, they are only available in the Nikon mount, so you couldn't use those anyway. But HSM lenses in the Pentax mount will focus faster than non-HSM lenses, so you might consider one of those if focus speed is important to you.