Tamron SP 300/2.8 IF AF

With this impressive lens you can't take a picture without being noticed. I bought it after finding the 80-200/2.8 not long enough. At that time I made mostly pictures of tennis and 200mm wasn't long enough. The FA100-300 was better but not fast enough. I Found this lens by a pdml.net user in Poland. It was in BGN condition and has some small damage by fungi's on inner lens-element. They seems to be dead because it hasn't become worse over time. It's a white version like the Pentax lenses where in those times (A* versions). This is my only lens with internal filters (with a odd size of 43mm). The tri-pod mount is big and steady. It has a mechanical focus limiter which works in both directions. Thanks to this AF is way better then any other Pentax lens for sport- and animal pictures.The focusring can be protected by another ring that is covering it. The MF ring isn't damped like the Tokina's one, so it's a bit more difficult to focus manual. But I mostly pre-focused  the action and wait for the subject being sharp in the frame. On the Ist-D AF-tracking seems to be better then on the Z1p and it useful outdoor. Indoor single AF does work, but tracking is way to slow. Subjective this is my best lens (even with the lens-damage). But I only use it when I don't need to travel a long way, It's very heavy ±2.5kg and big. You need a big back-pack or travelcase.

Performance
Optical this lens is a dream. I can easily say this is a PRO lens. Performance is already good wide open with only a bit vignettering on a full frame camera. On a crop camera it delivers pictures that are usable. With  the noise performance of today's camera's it's a dream. Stopping it down to f4 gives this lens performance that is good enough for very large print. Also the bokeh is great. If I'm right (didn't count) it has nine blades giving very nice pictures with out-of-focus background. Most of the pictures you see in the dia show are shot with this lens. Most other with the Tokina 80-200/2.8 and Pentax DA16-45/4.
There is a normal amount of CA at f2.8 (got a 8) which is much better then the Tokina 80-200. At f4 I can only see it in test-shots but not in normal pictures. If you want better results go to Nikon, Canon or Leica Pro gear. It's probably on par with the DA*300/4 if compared at the same f-stop. But the Pentax is much lighter and much easier to take with you.
This lens also works great with the Sigma EX 1.4x en 2x teleconvertors (TC). I was not able to test it because I didn't had enough space from my test-chart. Like most TC's they add a bit CA. But stoped down to f3.5 with the 1.4x and to f4 with the 2x gives me decent pictures. The 2x needs f5.6 to become tack sharp, but most lenses do with a 2x TC. I think this is the only lens I would recommend using a 2x TC. I don't have a 15Mpix crop camera to test it.

At this moment there aren't a lot of lenses available for Pentax in this class. Sigma has a EX 300/2.8 and Pentax a DA300/4. I don't know if you can still buy this lens in Pentax-mount new from Tamron, they still make a light updated version of this lens. They're not sold very often second hand.