As I expected Tokina is much softer wide open. Pentax is sharp even wide open., which is probably the best thing about this lens. The problem with the Pentax thought is that there is a lot of chromatic aberration, which is visible even at f/5.6, but finally disappears at f/8. The Tokina on other hand deals with the chromatic aberration much better and what’s more important even at f/4 it matches the sharpness of Pentax. At f/4 Tokina actually achieves more pleasant image than much more expensive Pentax. At f/8 both lenses look pretty much identical, but since these are suppose to be fast lenses, that is a bit irrelevant.
Pentax is probably more usable at f/2.8, but from f/4 I choose Tokina as the winner. F/4 is fine for me, I don’t need the lens to be completely wide open, so I’m glad I kept the Tokina and sold the Pentax. Tokina much cheaper than Pentax and much more flexible thanks to it’s convenient zoom range. Pentax is still a great lens, but probably a bit overpriced even for a vintage lens. I love great value for money lenses and Tokina is exactly than, so this is the one I highly recommend to anyone.
These are my opinions. I would love to hear yours.