Originally posted by pentageek
Have you considered 16-85? It's more expensive, but significantly outperforms 18-135 in terms of IQ.
Well, no it doesn't from 85mm to 135 mm the 18-135 significantly out performs it. Thats a range of 50mm. From 16-18 the 16-85 out performs the 18-135, that's a range of 2 mm, however it may be an important consideration.
The 16-85 is almost as bad at 16mm as the 18-135 is at 18
The 18-135 is better at 24mm
At 50 mm the 16-85 is better at ƒ4.5 and at ƒ8 the 18-135 is slightly better
At 85mm the 16-85 is better.
Pentax HD DA 16-85mm f/3.5-5.6 ED DC WR - Review / Test Report - Analysis Pentax SMC-DA 18-135mm f/3.5-5.6 ED AL [IF] WR - Review / Lens Test - Analysis
So in terms of "Significantly outperforms", no, definitely not. It out performs in some areas, but it is completely dependant on how you plan to use the lens. I use it as a walk around and as an all purpose lens it's better just because it's longer. The 18-135 is also centersharp through it's whole range. Meaning for portraits or pseudo macro etc. it's actually quite good, everything but edge sharpness in the long end. If you carry a 60-250 as well, that becomes irrelevant as the DA*60-250 is excellent centre and edge throughout it's entire range.
So where does the 16-85 excel? When you are in urban (or closed in) environments and wider is more important than longer and you are only taking one lens. The 18-135 is a much better all purpose walk around. It covers a much wider zoom range, more flexible, and fewer lens changes when it's on the camera.
The top lw/ph measured on the 18-135 are 2683 at 24mm ƒ5.6 and 2649 kw.ph, at 18mm ƒ5.6
The top lw/ph measured on the 16-85 is 2621 at 24mm ƒ4 and 2612 at 24mm ƒ5.6
Best to best, the 18-135 is better, make whatever arguments you want to make your claim but, that doesn't change the fact, someone else could argue the opposite just as effectively. Any advantage to the 18-135 can be made up for by taking the 15 ltd. The disadvantage of the 85 can be made up for by taking the 100 macro and the 1.4 TC. It all depends on what you value and what you can do without.
I have so little respect for the folks who bad mouth this lens, I hope it doesn't come through too much. I always feel like saying. if you want to promote your lens fine, if you have to put down mine, that's not acceptable. Especially if you just repeat the nonsense posted by reviewers who don't use the lens every second day.
The 16-85 is1:5.3. The 18-135 1:7.5. They are designed using different design philosophies the 18-135 in the long end being excellent centre sharp but not caring much about the edges in the long end, the 16-85 being better edge to edge throughout it's entire range except of course at 24mm where the 18-135 just surpasses it, and that's it's best focal length.
So what do you want? 1:5.3 better in a few areas but not in absolute sharpness, or 1:7.5 centers sharp every where excellent at 24mm. The 16-85 gives you better average overall sharpness at the expense of flexibility, and absolute sharpness. It's your choice, but it shouldn't be based on one being significantly better than the other. It's actually a predictable trade off. The OP has to choose which set of trade offs suits him. Personally I've never considered changing. Part of that is owning the DA*60-250. That lens is so much better than either of them, in the OPs position I'd just keep the 18-50 and get a 60 -250. After all, if the 16-85 is only bit more than the 18-135, the 60-250 is only a bit more than the 16-85 by that logic. Now there is an awesome lens. 1:3 zoom, constant ƒ4, excellent throughout it's range which nicely complements the 18-50. Lets get real. You'll take either of them off the camera to use the 60-250, at least I would,
http://www.opticallimits.com/pentax/479-pentax_60250_4?start=1