Pentax 18-135mm f/3.5-5.6 SMC DA WR Lens

I wasn't sure I'd like this lens from the mixed reviews, but after using mine for 3 weeks on my Pentax K-5 I'd say it's one of the best super zooms you can get for a Pentax DSLR.

Pros:
Weather resistant in rain, snow, etc.
Excellent color and contrast
Auto focus is very quick on my K-5 and quiet
Solid build, especially compared to other plastic extending zooms
Not a macro, but works surprising well for flower shots
Really smooth bokeh for closeups and telephoto
Reversible lens hood
Auto focus doesn't move the focus ring, but can be moved manually anytime for adjustments
Resistant to flare and ghosting

Cons:
Focus ring has no scale and can be moved past the "stops"
It's a varifocal zoom (zooming requires you to refocus a bit)
No aperture ring (unfortunately this is becoming a trend)
Manual focus ring is usable, but not ideal since it only turns 90 degrees (before reaching the soft stops)
Some chromatic aberrations (purple fringe) with high-contrast subjects (correctable with software, or in my K-5)
My copy shows slight visible barrel distortion from 18-24mm, good from 24mm-50mm, and slight pincushion from 50-135mm

Unfortunately this lens is not suited for serious video: where you usually want manual focus, zoom without losing focus, and the ability to rack aperture.

People also complain that the lens is soft -- a common complaint with super zooms. The real question is, "is it sharp enough." That's very subjective. My copy looked good, but a bit disappointing when pixel peeping. I tested it and found it was front focusing slightly. Easy to correct on the Pentax K7 and K5. Now it's sharp in the center, and sharp enough in the corners.

So as a travel/walkabout lens I give it 5 stars.
As a serious video lens: 3 stars -- you're better off with manual zooms and primes that give you much better control and selective focus.

With Pentax, there are not a whole lot of choices in production for a modern Pentax lens, especially for an all-around zoom. This 18-135mm is pretty much your best bet. Luckily, it performs decently well for its price. I have this lens and the 35mm F2.4 for really nice, leight-weight travel combination. The focal length of 18-135mm covers about 27 to 200mm 35mm film equivalent range, which is pretty good and covers many types of photos (landscape, portraits, group shots, and mild telephoto for wildlife). Plus the 1:4x magnification ratio means that you can also do product photography and close up of flowers. This lens has a little above average distortion at wide angle, so architecture photos are not going to be straight unless you post-process the image. The modern Pentax cameras can fix distortion in-camera too, but its slow. Vignetting, dark shaded corners, is about average..nothing I would concern myself with. Contrast is great a couple stops down from wide open. The color reponse of the lens is fantastic and typical of good Pentax glass. Its not too warm like vintage glass but great and vibrant nonetheless.

Sharpness. This is the subject everyone looks at. Pentax seemed to attempt keeping the longer focal length as sharp as possible. I feel they succeeded for the most part, at all focal length the lens performs very good a couple stops down from wide open. The CON is that the corners are very soft and blurry. I took a test shot of my carpet, pointing the camera straight down (using a bubble leveler to make sure), and at all focal lengths the corners are soft. Now, before one jumps ship, it is recommended that one read the Ken Rockwell article on the importance of sharpness. Most of the time your subjects are not at the borders of the picture, leaving the rest defocused anyway with bokeh, so most of the time when I compose my shots they are still very pleasing to my eye. And thats what matters. Only when you do critical work like close-up photography does a sharp flat plane work in your benefit.

For your money you get a lot of lens. Its lightweight because it doesnt need added elements for optical stabilization. It's weather sealed meaning it can handle mild rain with no problem (I wouldn't say torrential rain however). A feathered petal-hood is included and can be inverted backwards on the lens for storage. The focus ring is dampaned and quick-shift, meaning you can focus even with the autofocus switch on. Its located behind the zoom ring and has "soft" stops. The 62mm filter diameter is not so large that you must get expensive filters. The autofocus speed is not up to par to Canon's USM but it is about the same league as Pentax's SDM. Accuracy depends on what camera you have, but mine did well on my k-x. No back or front bias focus on my sample.

All in all, if you are looking for a one lens do-it-all, then this might be your best bet. If the longer focal length is not of concern, the older Sigma 17-70mm might tickle your fancy, plus its cheaper. Compared to the Pentax 18-55mm kit lens, though, it is way better and a good upgade. As with Pentax, if you want the best, you might want to look at their prime lenses.

From my experience with Pentax's superb non-zoom lenses, I had hoped this would be "perfect" also. But like most long-range zooms I've tried, it gets soft in the corners on some settings.
So for really critical work, I go for my primes, like the 50mm 1.4. But for travel and street photography, this is very good.
It's not fast, so Pentax's nice in-body stabilization comes in handy, for a bit slower shutter speeds.