Arkku posted a reply:
Ben Cooper: With an effective aperture of f/51, the diffraction is just as bad as on a lens with the geometric aperture of f/51 and focus at infinity. So, in a way, the aperture "really is" f/51. However, if the aperture on the lens was also f/51 and you still had that much extension, then of course it would be far worse. =)
According to the specifications, the "real" maximum aperture of that lens at 1:1 magnification is f/2.8. However, since there needs to be an amount of extension equal to the focal length to reach 1:1 magnification, the focal length is effectively doubled, and so you end up with an effective aperture two stops smaller, i.e. f/5.6. For the purposes of diffraction, etc, it's just like the real f/5.6 at infinity focus (zero extension).
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