Clive- I've seen what Drew's referring to. Years ago, a friend was having trouble getting sharp prints. She thought it was her magnifier, so I brought my Peak over, and we focused wide open. Then, closing down 2 stops, the print still wasn't sharp. Checking the Peak, at the chosen aperture, it wasn't sharp, and needed refocusing. It was a fairly cheap lens. She started focusing at the chosen aperture, without issue, even with her cheap magnifier. I haven't seen it happen with better lenses, though.