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.)
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.
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.