Me estaba volviendo loco con este combo, porque a menos de una distancia X, no podía sacar en f/2.8. Dicho en otras palabras, a cortas distancias el diafragma se planta en f/3.2.
Enseguioda pensé en lo clásico, limpiar contactos, sacar batería, etc.
Y encontré la respuesta:
I just purchased a used 60mm and my possible problem is that when mounted my aperture registers on my D300 from F3.2 to F36 rather than F2.8 to F32. Anyone experience this? Is it broken?
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The camera is reporting the effective aperture. The ring is marked with the aperture when focused at infinity. As you get closer focus the effective aperture becomes smaller. This is normal for all macro lenses.
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Pero ocurre sólo con lentes hechos en serio

You may find that the Tamron chip is just ignoring the bellows factor. The Nikon lens is correctly reporting the aperture for exposure light levels and diffraction purposes.
Bueno, quizá le pueda servir a alguien más.