I have a Nikon FM2 and I find the focus is slightly off at big apertures. For example, with my 105mm lens, at F2.5 I find even if the image looks perfectly focused with the split prism, on film it looks a bit out of focus.
I've tried this lens on my Canon 5d with an adapter, and it is sharp sharp sharp if focused correctly (the adapter has a focus confirmation, otherwise its impossible to manually focus!), so its not a lens problem.
Is it possible that the mirror in the FM2 is slightly off? If so, can it be adjusted? Or is it more likely that the problem is that I suck at manually focusing?