Originally posted by napawino i'm just talking about the "series" in general as compared to the other. Not a particular lens.
Is the SDN focusing system reliable?
What is "quick shift focus"?
How does the image quality compare?
I'm looking at upgrading from my 18-55mm kit lens and want to have my ducks in a row when I start shopping.
Well, if you're comparing the entire series against each other, it's important to realize the FA Limiteds are very... limited... in that there are only three relatively middle of the road focal lengths available (slightly wide normal to short telephoto). Taking the series as a whole, the DA* lenses can go from 16mm to 300mm.
The quality of the SDM focusing system seems to vary by lens, and is one of the most contentious issues in the Pentax world, with numerous reports of certain lenses suffering from SDM failure that requires repair. No official statistics are publicly available on failure rates, but in general people don't seem to complain about the focus
accuracy of these lenses. Some are reportedly quite slow to lock focus, though they will do so much more silently than the screw-driven FA Limiteds (which are themselves not the most lightning-fast focusers available).
Quick shift focusing is available on all the DA series lenses I believe -- it allows you to manually focus the lens to adjust the auto focus' decision WITHOUT flipping the AF/MF switch on the side of your camera. This is unique to Pentax's newer series of lenses.
Image quality will again depend greatly on which lens you're looking at. Both the DA* and FA Limited lenses are highly-priced premium lenses and any of them have the capacity to deliver quality images -- however, trying to directly compare them against each other brings in subjective arguments of rendering characteristics, as well as utility (for instance, an FA Limited can never match the quality of the DA* 50-135 at 135mm because no FA Limited exists at 135mm).