Comments about Sigma Sigma 28-70mm f/2.8 EX DG AutoFocus Wide Angle Zoom Lens for Pentax AF Cameras:
Stays on my K200 90% of the time. Zoom ring tight enough that I have never engaged the zoom lock. Highly recommended above kit lenses. P.S. I personally like real heavy glass for stability. This one could use an extra ounce or two, hope I'm not being too extreme.