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accordign to ken rockwell http://www.kenrockwell.com/nikon/60mm-afs.htm#perf
"The 60mm f/2.8 AF-S alters its optics as it focuses. It becomes slower as it's focused more closely."
and from
http://www.slrgear.com/reviews/showp...ct/1175/cat/12
The lens is marked as a constant ƒ/2.8 lens, but in fact the effective aperture will be smaller as you focus at shorter and shorter distances. The following table lists the aperture to distance differences:
Subject distance 6'' 7'' 8.5'' 9'' 10'' 1' 3'' 3' infinity Largest aperture ƒ/4.8 ƒ/4.0 ƒ/3.8 ƒ/3.5 ƒ/3.3 ƒ/3.2 ƒ/3.0 ƒ/2.8 Smallest aperture ƒ/57 ƒ/51 ƒ/45 ƒ/45 ƒ/40 ƒ/36 ƒ/36 ƒ/32
so still not sure WHY it does it. but that is the factor (at least for nikon)
heres a link to the tamron 90mm macro ( not sure if theres different versions) manual, i didn;t see anythign about it in a quick glance though.
http://www.tamron.com/assets/pdfs/272E.pdf
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