<p>Sorry to read about your disappointment! I also own that lens since six years back in time. I use it as my complementary telezoom when travelling, together with my Pentax 16-50mm/2.8 normal zoom. I also have the Pentax 60-250mm/4, which I prefer when not having to consider weight and size restrictions.<br>
The 50-135 zoom gives good picture quality (above medium quality I would say): I have never had any obvious problems with it regarding sharpness or the kind of exposure issues that you describe. The 60-250mm is however the better one of the two lenses, really giving fantastic picture quality in terms of sharpness. Both are very quiet when zooming which is convenient but I had to repair the SDM of my 50-135 when it stopped working once. The SDM is a weakness, I agree. The SDM of the 60-250mm, however, seems much more rarely affected.<br>
I think you got a bad copy so I recommend you to return and change it. I did so with my first 16-50mm and got a good copy instead. But the 16-50mm zoom also has had the SDM problem, twice in fact. One of these times they fixed it for free, I am happy to say.<br>
You can always go to Flickr to see some of my pictures. I have started to use an account there for picture presentations. They are still rather few but I have a couple of winter videos and a picture series from Iceland.<br>
www.flickr.com/photos/97622656@N05/sets/<br>
Wish you good luck with your lens problem!</p>
