I think I was correct about the focus issue being down to me hanging on to the lens. Here's two of the most boring shots of my back garden, one (slightly underexposed) at f5.6 and one at f32, without touching the focus between shots. The focal point is the little sheet of stars shown in the detail images.
F32:
F5.6:
Ignoring the underexposure, there doesn't look a lot wrong with the focus (done with a cable release both times!). These are untweaked, straight from the scanner.
Neil
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