There appears to be a problem with the Pentax version of this lens. Under 31mm it is communicating the wrong SR Focal Length by a very large margin. This introduces blurring when SR is on. Please check the following thread about it on PentaxForums.com. It has been verified by a number of users with the lens on different Pentax models. http://www.pentaxforums.com/forums/...y-compatibility-problems-shake-reduction.html If you own the lens, please check it out. Also, I would suggest that you contact Sigma to inform them of this problem so that that can issue a firmware fix to send the right SR Focal Length.