No more than 500mm. The 600/4 and 800/4 are big, heavy, expensive and prone to chromatic aberrations. The M* 800/6.7 is much better but even more expensive. The 1000/7 or 1000/8 reflex probably only exists in a display case back at Pentax HQ. The M* 300 and M* 400 also fall into the category of too expensive, but I can live in hope of fluking a bargain one day.