Question for users of the 100mm USM Macro: When looking through the lens while it's off the camera, and turning the focus ring, does it go from a "wide open" perfect circle to a more octagonal shape that almost looks like slightly stopped-down aperture blades? That's what mine is doing, and I wonder if it's faulty or if that's normal for this particular lens. At first I thought my aperture blades were stuck, but that wouldn't make sense given the wide-open change at one end of the focus throw. And when I mount the lens, stop down to f/4, and press the DOF preview button, the VF does get darker.
Isn't it that altough its a prime it is actualy changing focal length in order to get the magnifacation at the short end. IE its not always 100mm. so its actualy changeing the apature to keep the ratio to acheive the 2.8 Fstop.
I may be completely wrong, it happens (wife thinks it always happens ) but if you look at a constant apature zoom when you zoom from one end to the other you can see what you describe
(wife thinks it always happens 
) but if you look at a constant apature zoom when you zoom from one end to the other you can see what you describe