Originally posted by Adam Yup, that's normal.
As for the AF adjustment issue, -10 is generally a bit much, so I would exchange the lens if possible. However, before you do so, please ensure that you performed your tests in a controlled environment, as you don't want to be sending back a lens that has nothing wrong with it! If it ends up only needing -3 or -4, then I'd hang on to it.
Here's a guide:
Fixing Front and Back Focus - Introduction - PentaxForums.com unfortuantely, my lens was a 2nd hand purchase. I did my little focus experiment in my house at night, with a tripod, selftimer, room was bright enough I guess since the meter registered ISO 400 with f1.9 and 1/80s. Since the focus is acceptable now, I will worry about future lens in the future
, if worst thing happen, I'll just get pentax to calibrate my system.