i have no personal experience with this lens
have you seen the In Depth Review on that lens
Pentax-DA* 50-135mm F2.8 Review
Pentax-DA* 50-135mm F2.8 Review - Introduction | PentaxForums.com Reviews
AF Verdict
The autofocus speed of the Pentax 50-135mm is perhaps its weakest point. Many will plan to use this lens for sports, and for this purpose the 50-135mm simply does not deliver. On the other hand, for other applications you will probably find its autofocus to be sufficient.
One interesting observation we made is that there's some latency between the time that the user activates the AF and the time that the SDM motor actually starts to move, which might account for some of the perceived sluggishness.
The fact that this lens has silent autofocus to begin with, and that it supports quick-shift reduces the overall severity of the slow AF speed in our eyes, though it's still something that sports shooters should consider when deciding between this lens and other options, such as the Sigma 70-200mm OS HSM, which boasts much faster AF.
Read more at:
https://www.pentaxforums.com/reviews/pentax-da-star-50-135mm-f28-lens/autofo...#ixzz5AJGZJG00
Disabling SDM to use Screwdrive
If the SDM in your 50-135mm fails, it is possible to deactivate it and force the camera to use the screwdrive mechanism. However, if your SDM has not failed, then we do not recommend disabling it. The screwdrive AF system is much more noisy than SDM, and user reports have shown that it is not faster in practice.
Read more at:
https://www.pentaxforums.com/reviews/pentax-da-star-50-135mm-f28-lens/autofo...#ixzz5AJGSevIu