Thanks for the replies all. Glad you found the test useful and interesting.
One thing I should make clear is that I had no illusions that the Tamron would be better than the DA 15. Of course the DA 15 would be better. Also, to answer a question, the pictures are just JPG's straight from the raw files with no processing done other than the format conversion.
I tried to add some post processing today to the Tamron picture, to see how close I could get it to the DA 15 picture.
Long story short: I could not. I couldn't even get it close to the sharpness of the DA 15 pic, and I also could not get the flare out of the picture with the clone stamp tool. Now, this is just perhaps a mark against my PP skills, as someone with more experience and/or better tools at their disposal (Elements, for example. All I was using was Lightroom 4) could probably do a better job of equalizing the pictures. Nevertheless, it's a definite mark against the Tamron 17-50.
In stating that, my goal was to see if I could get "90%" of the way there, and I feel like, barring the flare which with some careful measures can be avoided, the Tamron can reach that goal, and in some cases even exceed it. The DA 15 still has that "je ne sais quoi" factor though, and it should be a cornerstone of any Pentax prime-based kit.
Rondec: I would rank lenses for performance between 15mm-18mm as follows:
DA 15 Ltd (By a full head. Should not be a surprise)
Tamron 17-50 (The new poor man's DA 15)
DA 16-45 (The old poor man's DA 15)
DA 18-55 (L or WR, surprisingly good)
DA* 16-50 (Too much barrel distortion, kind of soft wide open hurts it for astroscapes)
DA 18-250 (Mush at any aperture)
I haven't tried any other lenses, but that's my assessment based on using the above.
Last edited by EarlVonTapia; 06-04-2013 at 12:23 AM.