sorry to say that I don't have any Pentax 300's, the only Pentax I have is a 50mm, but it has the same problem, less pronounced because of the larger dof, but still has it. I have to send the camera in, but because this being my only body find it very hard to do. :roll:
I'm using center focus, and the thing is that when focused with f7.1, and then changed to f4 (without refocussing) the object stays in focus, the other way round it doesn't even get into focus. The strange thing is that when using a focuschart (printed one a bit larger A3 size) is that when focusing on the line you seen the backfocus at f4 slowly getting into focus at f7 and then going more towards a frontfocus above F11 but because of the large dof it keeps the focus.