Hi, Ive just got myself a second hand Pentax 300 f 2.8 (manual focus) lens + two Pentax TCs ( the 1.4X and 2.0X L series) that Ive used in Namibia. Im very satisfied with the 300mm lens (but less with the combination 300 + TCs). Im interested in taking pictures from a car (in National parks like the Kruger N.P) so weight isnt a big issue. I was planning to sell the two TCs in the future and add a second hand 600mm from Pentax (f/5.6 or f/4, depending on what I find and on costs). A local store is selling a second hand Pentax 250-600mm f/5.6 manual focus (with visible scratches on the outside) for about 3.500 USD, so I would like to know if anybody knows how this zoom compares, as far as optical quality is concerned, to the 600mm, and eventually would it be worthwhile loosing something in quality but gaining the versatility of the zoom lens. Thanks for now.