Originally posted by audiobomber Why not get a Pentax 55-300 and crop to a 500mm fov with all those K-7 pixels? The image will probably look better than you'll get from some old dead cheap 500mm hollow tube.
I'd say based on my tests with the Quantaray "cheap hollow tube" that it would probably behave *slightly* better than simply cropping, at least when doing large prints, or crops from that crop - just as my cheap mirror lens does when comapred with crops from my 50-200 on the 10MP K200D. But at typical print sizes, or just viewing the image on screen, you're right - the 55-300 undoubtedly will do as well. And the 55-300 would be a stop faster, have more DOF, support auto focus, and be a fraction the size.
Here, BTW, is the one sample I had uploaded a while back. First the (resized) full image from 600mm:
And here's a 100% crop from my 10MP K200D (14MP cameras will of course expose the flaws of the lens that much more readily):
This one is not bad, really. Other shots showed CA much worse than this one.