Originally posted by Hayashi believe 18-50 RE must be doing a better job right ?
No, the 18-50mm RE is not a better lens than the DA 18-135mm, in fact quite the reverse. But for compactness, and with good quality for a kit lens, this lens is very unique. For very critical work there are better options, but which are not nearly as compact.
Vignetting has been tested as being heavy with the DA 18-135mm, but only at its widest 18mm setting and then only at wide open aperture. When stopping down, vignetting goes down rapidly. Vignetting performance is actually quite good for a zoom lens, especially one of such a large zoom range. Even very expensive, highly-regarded lenses can exhibit considerable vignetting wide open, like the FA 31mm f/1.8 Limited, for example. This problem is also greatly reduced with this lens by stopping down some.
As to image quality, the DA 18-135mm lens can deliver excellent sharpness figures virtually throughout its zoom range in the central portion of the frame. Edges of frame sharpness are very good to excellent also between 18-70mm, but need to be stopped down a little. Edge sharpness diminishes as the lens is zoomed into a more tele range, but this is usually less important in longer tele shots anyway. This is exceptional performance for a lens of such versatility and zoom range. Many fine images are presented in the "DA 18-135mm WR, Show us what it can do" thread as testimonials to its impressive capabilities, which can show the imaging improvements as Pentax DSLR cameras have progressed over time. This proves the quality of this lens, revealing the improved quality of the latest Pentax cameras. I've had mine for over 10 years, and have used it on numerous camera bodies. I just had it on my KP for an event shoot a week ago.
I also have a Sigma 17-50mm f/2.8 EX DC HSM, along with my Pentax DA* 50-135mm f/2.8, which are excellent on my KP, and I love them when I don't mind carrying their relatively modest additional bulk, and the obvious need for lens-changing, and especially when I will need their f/2.8 advantage.