Originally posted by tibbitts I have not been able to tame AF on that 17-50. I can correct it (though it needs all of the K-5's 10 adjustment range) for one focal length, but even then I'm not sure it works at all subject distances. And it definitely doesn't work for other focal lengths. Tamron took two tries at adjusting mine as well so I think viewfinder AF might just not be possible. I purchased a Sigma 17-50 from the PF marketplace, and it does better with AF, although I still have some difficulty with it at the shorter focal lengths and longer subject distances. The Sigma seems to have a flatter focus field, and I can manually focus it fairly well, so I'm very happy with it. The focus ring does have a little of the Pentax 16-85 jumpy image effect, just enough to be annoying, but much tamer than on the 16-85.
My 16-85 has very good image quality and for my uses it's an excellent lens. I had trouble finding a relatively good copy, and even the best one that I kept still has some annoying quirks, but overall I've been very pleased with it.
Interesting, I read your posts on the Tamron 17-50, and it makes me think of all the issues I've had with their 10-24. I have a 17-50, and I never had any issues what-so-ever using with my vanilla K-5. I loved that combo so much, I got the 10-24, and I was never happy with the lens. I always wondered if it was the camera or the lens. I don't think I've ever used either much once I went to a K-3 because I shifted to relying more on prime lenses and found that I never really used the 10-24 any wider than 15 mm.
I'm glad you found a solution.