Originally posted by pop4 You're forgetting the other brand, Canon, and some of their L lenses. How come they can make lenses that are dust and weather resistant (but not water sealed), and still have USM focusing that's fast and quiet, and doesn't self destruct like SDM seems to do...
I was just going to mention that owning L glass I know they are WR. The thing is though... unlike Pentax Canon does not offer WR on all of their L line.
For example the 17-40L is WR ONLY if you add a filter to the front of it (odd design, but the barrel of the camera is static, and the end element floats and moves inside that barrel). Otherwise its just dust sealed.
The reality is its hard to compare against the Canon, but they all use USM and offer dust resistance. They don't seem to have an issue.
From what I have read about how Pentax seals the glass... is that it uses felt/cloth O-Rings inside the barrels and focusing points (basically all the parts that need to move/rotate within eachother)... and the actual elements are sealed off completely from water/moisture, which I am guessing involves rubber seals and cementing. The cloth O-Rings don't cause enough friction for me to believe its wearing down the SDMs.... however maybe the SDMs are just so weak that any additional friction does cause it to fail early. The Canon's and Nikon lenses use a very similar type of dust sealing as well.
Of course I could be completely wrong.