Auto Focus Hocus Pocus

Hi gang! here's one for your little gray cells. Beloved Tamron wide-angle stopped focusing in auto-focus on beloved K-5. (AF works fine with Pentax wide-angled light lens). Sent for repair came back same after couple of shots. Endless groans as going on holiday. However, and here's the thing - playing about, I found that if I use the Autofocus Button on the back of camera instead ... it works. Anyhoo, I guess it's one for the boffins at Tamron and/or Pentax. However, I was wondering if anyone could throw enlightenment on this anomaly please?

k-5 menu section 5 button customization, shutter button half press, enable AF,
will do the trick,in the same sub menu AF button ,enables the AF button or Cancel AF

Hi, thanks for the suggestion. The shutter-button-half-pressed enabled autofocus works fine with the other lens, and did work fine with the Tamron too - it's the only way I've ever focused. It's just that with no warning the Tamron lens suddenly started not to focus at focusing at shutter-button-half-pressed even though the shutter-button-half-pressed was enabled.

But I discovered that cancelling the autofocus at half press and as I say just using the back auto focus button works. Don't know what's happening there.

i wonder then if you need to clean the contacts on that lens, i would check them against the other lenses you have as well to see if there ay difference...

Thank you so much Caz. That seems the way forward. I'll finf out where the contacts are and check that on my return from holiday.

The contacts are those little shiny bumps on the camera side of the lens mount, rub them vigorously with a lint free cloth, same goes for the corresponding contacts on the the camera body's mount