Reviews and Problems with Sigma 50-200mm F4-5.6 DC OS HSM
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Sigma 50-200mm f/4.0-5.6 DC IF SLD Optical Stabilized (OS) Lens with Hyper Sonic Motor (HSM) for Pentax Digital SLR Cameras
19 January 2012
Summary: Unfortunately, the vendor did not fully test compatibility of this lens with all Pentax cameras. Thus, while it appeared to be a nice lens, the autofocus function was inoperable on my Pentax K10-D. I thus returned the lens since the manual focus ring on the lens did not operate smoothly.