Have to admit I'm struggling with this a bit. Maybe I need new eyes, or a new brain.
What exactly is the critical difference between shooting with a 150 and a 300 that makes it impossible to get a sharp image with the 300, whilst the 150 will produce amazing results?
Howard, you're not saying there's a subtle or gradual change here. You are claiming that "amazing results" are possible with a 150mm lens, and "nothing helps to produce critically sharp images with the 300mm".
Is it the quality of the lens itself (and are you referring to the /4.5, /2.8 or both?), or a fundamental limit based on some function of its focal length, the camera, and digital back?
I can't see how it can be anything to do with the focal length. Assuming a subject lit in direct sunlight, you'd be looking at, what, about a 1/1000th or 1/1500th of a second shutter speed at f/2.8. And that's at ISO35? That's plenty fast enough to freeze your subject hand-held without worrying about mirror slap or shutter bounce, surely?
Conversely, the Hassy HC300 is f/4.5, so you're already over a stop slower with that, not to mention limited to 1/800th shutter speed anyway.
Is your point regarding technical challenges around the focal plane shutter and mirror of the DF, or simply that the HC300 is a far superior piece of glass?