I have this old Pentax K 500/4.5 lying around for some time. It is an original K mount lens, not the M42 version. Yesterday I took some shots in the garden, in comparisson with the Bigma.
The good news is, that sharpness and contrast of the old Pentax stopped down to around f/8 (to get at least a bit of DOF) is still very acceptable, even on the K20. Indeed much better, than I expected. The Bigma is visibly softer at 500mm fl, but otherwise nice and sharp and with very good colour rendition.
But the K 500 shows its age in form of CAs clearly.
I have attached three examples:
1. the full shot, just reduced in size and a bit of LightRoom curves adjustment
2. a 100% crop without fringe removal
3. a 100% crop with fringe removal fully applied = LR cannot remove the red/cyan fringing.
Ben