Originally posted by slip There basically your for volleyball shooying at f/4-4.5 a guy that shoot some plane in bright light at f/5.6 with lot of dof and me that say the 50-135 would perform better on a D7100. This isn't exactly that convaincing.
Don't get me wrong you can shoot the olympics with a full manual camera or why not even a chamber. By pre-focussing, shooting when the subjects don't move, by increasing dof with smaller apperture and by shooting a lot you'll manage to get a few keepers. And practice will also help you a lot.
This doesn't mean that the gear is the best suited for the type of shooting you do. A sigma 50-150 already perform significantly better and there one cheap used available not long ago on the marketplace. Maybe it is still available. K3 and K3-II improve significantly over K5 but we are still not at Canikon level here, by far. And because the Nikon DSLR and associated sigma zoom are not more expensive than in Pentax mount, if action is a major concern to you, it can be be really worth to consider Nikon as a better alternative.
There nothing bad with trying to get the best tool for the job.