Originally posted by Rob22315
I too got poor focus results from my Pentax 80-320 zoom at the long end (~250mm+). My theory is that it's a combination of the smaller aperture and the long focal length that creates the problem.
Less than about 250mm, the lens is very sharp, the focus reasonably accurate.
Perhaps longer focal lengths and or smaller apertures are factors but in this case at least there has to be something else. 2 lenses, an older sigma, and a newer sigma, both at 300mm and both at f5.6. One auto focuses fine, the other doesn't. That is using the same target and a flat focal plane to avoid the possibility that it is grabbing something unintended to focus on.
Since the problem does seem to be at the long end perhaps the long focal length and smaller aperture both weaken its ability to focus accurately but doesn't quite completely eliminate it. Perhaps there is another factor (lower contrast glass seems like a feasible possibility) that pushes it over the edge where it can not focus right? That might explain why one lens can focus at 300mm f5.6 but another can not. I strongly suspect that longer focal lengths and or smaller apertures play a part but I think there may be another factor at play.