Originally posted by bertwert 
Try cleaning the contacts maybe?
Yes, and contact is good. The AF distance indicator moves over the whole range from near to infinity.
---------- Post added 21-01-16 at 06:17 ----------
Originally posted by r0ckstarr How long have you had the lens? How often do you use it?
I've had the lens since 2014. I used it about 5 times a year. The AF would not lock 100% of the times, even when I got it new; At the time I also used it with a K-5 and it also behave the same way but locked less often compared to using it with the K-3. But it worked acceptably well. However, now, the number of occurrences when it can't lock focus has increased a lot. Now, I miss a number of shots everytime I go out for a shooting session, so it becomes annoying.
---------- Post added 21-01-16 at 06:19 ----------
Originally posted by darylk Does the problem happen only when trying to focus near infinity? If so, check your fine focus adjustment to make sure the camera isn't trying to make the lens go past infinity, in which case the focus will not lock in.
Focus at infinity is Ok. There's some mechanical play around the focus point, so the AF distance moves back and forth around the focus position, sometimes the camera (K-3) is able to lock focus, sometimes, AF lock fails and the green indicator is blinking, so in AFS mode / focus priority, the camera won't take the picture, I have to do another AF sequence in hope that the K-3 will be able to lock focus.
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