Leigh, it is good to hear with someone with Nikon repair experience. I can concede operator error, but only on the assumption that as operator I assumed the split screen would give a greater accuracy than it does. Others have suggested that the lens may change focus as it is stopped down; I have noted that the split screen only agrees with the ground glass at the widest apertures; and some have suggested I really get to know my lenses to compensate for system limitations. I am beginning to think that the last point is important - whether it is me or the lens, I am going to have to focus slightly beyond the subject at medium distance/aperture, for optimum results.