Originally posted by philbaum Tom, you are spot on with that comment! I first experienced the difficulty shooting the 300 with a 1.4 TC on a grass slope with a Manfrotto carb. fiber tripod, no spikes. The cityscape i was shooting was uniformly soft over the 20 - 30 shots i took. (these shots were all taken with 2 sec delays) Went back a few days later, forced the legs by hand into the slope, hung a heavy bag from the tripod, and the shots were magically sharp. As you say, my SMC 400 has the same problem. Later on, got a heavier aluminum tripod, giottos, with large spikes. much better to shoot from, but you can see the 300 and 400 lenses just sort of quiver around the tripod axis. There's a rotational momentum problem there that one just doesn't see with a smaller length lens.
I doubt that some of the reviewers of the 300 knew enough to properly test a longer lens. I have a friend in our camera club that had problems getting his new 600mm Nikon lens, ($10,000 by his report) to be sharp, i suspect for the same reasons.
I have a very sturdy carbon Gitzo, but that is not quite enough (no spikes), nor is my Berlebach 3032 (which has spikes), as that can vibrate at times. But with a nice sharp lens with built-in anti-shake, like a Nikon AF-S VR 80-400 Mark II, the problems is far smaller than with my K-5 and my 400/5.6. Sadly the amazing Nikon lens's VR only works with Nikon cameras (well there is an adapter for NEX, and MFT, I think, that can power the VR), but Pentax owners are out of luck!
I'd love a modern Pentax that could use modern Nikon lenses ;-)!