Originally posted by corvin So many good answers so quickly.
WR is fairly important to have in at least one versatile lens for me...eventually.
Size/weight is a non-issue. Irrespective of everything else, I'll have the Tamron 90mm macro and the DA 50 (f/1.8, i think) in the bag with me always...in addition to whatever else I buy.
I'm terrible with post-production editing (so far) so most of my stuff gets retouched little (most of what I do is either in paint.net or rawtherapee).
I live in Utah so I have a wiiiiiiide range of landscape/light/seasons open to me so I'm thinking a lot about versatility.
Speed of the lens can get important quickly...which is why I was thinking about the 18-135 WR paired with the 2.8 of the Tamron 17-50 (or should I be thinking even faster?).
OK... I have the pairing you are talking about. I have not fully tested it on my K3, yet, but they both got used heavily on my K5. For landscapes, 2.8 will be fast enough. The Tamron lens is so wonderful. I used it almost exclusively on a roadtrip through Utah (with a K10d) 5 years ago.
I got the 18-135 for the larger range of focal lengths and the WR. The lens is somewhere between the Tamron and the Pentax 18-55. I use it a lot because I can get away with it as the most minimal kit possible (i.e. camera and mounted lens). But, I'm never 100% happy with my landscape shots as I am with that 17-50 Tamron.
With the K3 and it's lack of AA filter, the 18-135 may be better, but so would the Tamron.
For WR, I don't know just how important it is. Maybe if you lived in Seattle. I went for many years with the Tamron as my primary lens. That included shoots out in some rain, windstorms in Arizona, etc. I never had any problems with it. I used it throughout Iceland leaving the 18-135 home. Of course it isn't weather sealed, so your experience could be different, but I do think these lens do ok as long as you aren't going crazy with them zooming a lot or purposely standing in the pouring rain.
Back to your situation. The one thing I didn't like when I was traveling around the SW was that there are many places where I couldn't get wide enough. 17 mm didn't always cut it. I was ignorant of that possibility. My solution then was to stitch photos together to make up for reach. Specifically at Zion. Later on I got the Tamron 10-24. It's not as fast, and really not that sharp either. It is a good lens and it does solve the issue of wide conditions, but it's also a little big. However, I've since replaced that with a 15 mm and 21 mm prime, which could actually replace both Tamron's although I'm not convinced that they exceed the 17-50 mm lens anyway.
Anyway, you might think about your needs and where you'll be shooting. Knowing the weather in Utah, I don't think you need to worry that much about WR lens. With the scenery, you should emphasize sharpness. You have a longer focal length available with the 90 mm lens and even a 50 mm lens. You might want to focus on image quality and go from that be it the 17-50 and another lens (a wider one perhaps) or even perhaps the 16-50 or wait for the 16-85 and reviews to follow. You have a lot of great stuff to photograph, might as well make it as perfect as you can.