Originally posted by Ash Why do you say this Paul, except from your own experience of a dubious copy of a 55-300? Not even theoretically can you prove the 55-300 not being of better optical quality than Pentax's 50-200s.
The 50-135 and 50-150 offerings are a completely different story, but out of the question for the OP.
I would suggest that the 55-300 is more than you will need for non-low-light critical conditions, particularly in the 55-200 range.
The 55-300 is the second lens for my k100 and k200 that has had serious performance deficiencies.
I've had two cases where lenses obviously performed differently across the frame - top to bottom, or left to right. In the first case, I traded a lens that had outstanding performance on one side, for one that has fairly consistent performance everywhere. But the replacement never had the resolution that first one did in portions of the frame. I didn't have a third similar lens to compare, so I really don't know if the second one I have is "good"; I only know that on balance, it's better than the first one. Should I have gotten a third, fourth, or fifth copy to compare? How is someone buying a lens - without a great deal of experience - supposed to know if they have a good copy, if they don't have at least one identical, or at least competing, lens to compare it against? I'm guessing that not all lens defects manifest themselves as differences across the frame. Maybe some lenses are just slightly poor everywhere, but I don't know how someone would be able to tell, without any comparison.
When I had film cameras, I compared as best as I could my SMCT primes from the '70s against the Canon zooms I replaced them with in the '80s, and with the tools I had available to me, I couldn't detect a difference in my kodachromes, nor could I detect a difference across the frame (except for sharpness uniformly falling off toward the corners.) I was satisfied with all those dozen or so lenses. It may be that today we just have so much better ability to compare performance; or it could be that lenses were just more consistent then than they are now, possibly due to today's more ambitious designs.
Not having a statistically significant sample by any means, I have to consider it possible that quality is so inconsistent that "theoretical" optical performance is irrelevant, so even if I had some knowledge of optical designs that I don't, there'd be no point in trying to point out which lens should or shouldn't be superior.
Paul