The Tamron 28-75 f/2.8 is a very good lens
for the price... However, all three Tamron lenses I've owned for Pentax have front focused significantly (when none of my Pentax lenses required any more than +/-2 adjustment). I returned two copies of the 70-200 f/2.8 to my supplier because neither of those could be calibrated within the +/-10 AF fine adjustment; at least, not so they would focus well enough at all focal lengths both at distance and close in, wide open. And my 28-75 f/2.8 is currently with Tamron UK's service agent for alignment, as - after a fair bit of use - I've finally realised that it, too, front-focused beyond the limits of the camera's AF fine adjustment (especially at close range). I'm looking forward to getting it back, as - overall - I do think it's an excellent lens
considering what I paid for it.
Sony's AF is exceptionally good, though, and the Zeiss lenses are just gorgeous... My Sony A99-based Hasselblad HV and Sony Zeiss 24-70 f/2.8 combo focuses incredibly quickly and with unnerving accuracy - I honestly can't think of one occasion yet where it has missed critical focus; very impressive indeed. So, yes... Sony's AF is streets ahead of Pentax. But then again, Pentax wins hands down in other areas... Bring up the shadows in high contrast areas on your Sony images and you'll find nasty, pixelated noise along the dark edges, due to compression in the .ARW raw files. This has completely ruined a couple of my photos which, had I taken them with one of my Pentax cameras, would have been perfectly usable shots. The dynamic range and clean shadow detail that can be recovered from my Pentax images is just remarkable compared to Sony's compressed raw files.
As always, with any camera and/or lens, it's a case of understanding the strengths and weaknesses, and working with those