In article <4f3dq2$9i0@rtpnews.raleigh.ibm.com>, levenson@vnet.ibm.com says...

>I was shooting with my new Nikkor 105mm f2.8D micro lens today.
>(...)I noticed that closer than 10 feet and the aperatures start to
>change. At the closest focus, minimum aperature changes from f32 to >f64 and maximum aperature changes from f2.8 to f5. Is this a result
>of the micro optics kicking in? I didn't realize that the aperature >was variable like with most of the zooms. For a while, I thought the >cold was freaking out the electronics in my camera.

With a standard-construction lens focused to 1:1, you would see
2 stops of speed loss. Since the f2.8 105mm Micro-Nikkors are
variable focal-length lenses to shorten the very long extension
that would be needed to reach close focus if they were not, the
speed loss is less than the normal 2 stops, but, of course, much
more than zero.
Hope This Helps