There seems to be a problem with my DA L 55-300mm lens, with very soft output at or near the telephoto end:
Wide-open test shot
Pentax K-5 with Pentax DA L 55-300mm @ 300mm. Av, 1/320s f/5.8 ISO 125. 100% crop from center. 
Stopped-down test shot
Pentax K-5 with Pentax DA L 55-300mm @ 300mm. Av, 1/400s f/11 ISO 500. 100% crop from center. 
Both of the above images were taken in Live View with contrast-detection AF, and camera shake does not explain this behavior (the camera was hand-held with image stabilization on).
This does not appear to be normal, even for a consumer-grade telephoto zoom lens. Is the lens broken? If so, is it worth fixing? (Note that a new DA 55-300mm, with a metal mount, Quick Shift Focus System [full-time manual focus], better construction, and distance scale can be purchased for US$359.95.)
Edit
Upon further testing, this seems to be an intermittent problem. Sometimes, the lens would seem normal, but the problem would appear later. The lens appears OK (but somewhat soft) around 150-200mm, and shows no obvious issues below 150mm.
Here's a clearer example of what's happening:
Brick wall test
Pentax K-r with Pentax DA L 55-300mm @ 300mm. Av, 1/6s f/5.8 ISO 100. 100% crop from center. Camera on stable surface. 
Edit 2
This isn't consistently reproducible. Most noticeably, the problem seems to clear up when I zoom out, then zoom back in. When the lens is kept near the long end, spherical aberration, coma, and chromatic aberration eventually become unacceptably high. The problem isn't normally noticeable at shorter focal lengths. It seems there is a damaged or worn support structure inside the lens.
Answer
There seems to be a problem with my DA L 55-300mm lens:
Pentax K-5 with Pentax DA L 55-300mm @ 300mm.
Av, 1/320s f/5.8 ISO 125. 100% crop from center.
If it didn't do it before and it does it now then somethings broken - probably. But, you know that.
Is that hand held or on a tripod? At 1/320s and 300mm you are on the reasonably comfort bottom limit of what can be hand held wit reasonable care without stabilisation. (ie you can get to say around 1/150th with super stable hand, good experience and care and Ninja breathing).
IMPORTANT:
- If using with a tripod **turn the antishake off* !!!
- If using without a tripod, turn it on.
That certainly is not pretty and I'd hope for far better from even a consumer lens, properly used. Full zoom is going to be less than its best setting. So you could start out by giving it a less challenging task to see if it is grossly broken or works OK at some settings.
I don't know the specific lens but at about say 150 mm, f/11 and on a tripod, I'd expect it to do about as well as it is capable of. That would give slightly under 1/100s at the same ISO as above, so you'd want a tripod or have the camera solidly held on a stable mount. (I chose 150mm as the ~ geometric mean of 55 and 300mm - and longer zooms usually work at about their best around mid range.
These enthusiastic user reviews suggest that your should give better than that.
Samples at 300 mm here and here
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