[backcountryskier]

  I had the Tamron 17-50 F2.8 non VC in the Pentax mount. It took me a while to realize why the lens front focused.

 

    After having calibrated it, when shooting fully open the lens was accurate, and from F2.8 it was accurate all FLs and all apertures up to F5 ....as soon as you stopped down to F5.6 or more it front focused........something in the AF algorithms within the lens created this problem, the focus ring would turn about 2mm more front focused, than when F5 was selected on the camera or higher.

  I posted here about it asking for anybody to try this test.....no response.....maybe nobody here had the lens.

    I also posted my results on DPreview Pentax forum, and Bingo, five or six people ran the test and found the same result!

  Many suggested including Klaus that maybe it was there to counteract RSAs, which would seem reasonable..........but it wasn't that......it was a flaw in the programming maybe confined to the K mount, I never looked further at other mounts.

   Once I found the problem I rarely stopped down to more than F5 and all shots were in focus.

   

It was the "sneakiest fault" though, so anyone having front focusing issues with the lens should try the test......check whether the front focus is confined to apertures smaller than F5. 

 

 

 This is of course PDAF though the viewfinder!

Thanks for the suggestion!  The problem exists at f/2.8. 

 

If I look through the lens, point to infinity, and repeatedly half-press the shutter button, the lens will refocus at seemingly random spots, beeping every time for autofocus confirmation.  I've gotten used to keeping a finger on the focus ring and sensing whether it turns to the right, towards infinity.