So I finally broke down and went with the Pentax long zoom after spending months agonizing over the choices. In the forums I have read that many people have had focus problems with the Pentax cameras and both the Sigma and Tamron long/fast zooms (70-210 f2.8). I debated the merits of manual focus vs. autofocus. Finally, I found a very good deal on the 50-135, and the Pentax brand won out.
The win is very well deserved. After just a few tens of pictures, I can tell you without any reservation that the lens I have is ridiculously sharp and contrasty. It focuses much more quietly than the 'screw drive' lenses, but certainly not much faster. Its focus is, however, both more accurate and more sure than any other autofocus lens I own. It's also the sharpest zoom lens I own, even approaching the image quality of my Tamron 180 f2.5. It's definitely as sharp as my old Pentax-M 135 f2.8 (which I shall be selling on ebay soon, as a result).
The inset here should give you some idea of the resolution this thing exhibits. It's visually indistinguishable at 2.8, as well - wide open! The corners are slightly less sharp at 2.8, but by f4, I can't see any visible difference between the center and the corner. I'm sure there's a measurable difference, just not an obviously visible one.
The lens isn't very heavy as such lenses go - certainly nothing like its 70-210 brethren in size or weight. It handles well and fast, and on the APSC-sensor'ed Pentax K20D, it's comparable to the 70-210 type lens on a full-frame camera. It's also weather-sealed to match the K10D/K20D, so you can go out in the rain and shoot pix... I'm not that brave, but it brings me some solace in case I get caught out in the rain.
I like the lens so much, I'm thinking about selling off a bunch of other stuff so I can get the 16-50mm f2.8 sister lens to complete the pair.
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