Sigma 105mm macro F-stop reading

I am currently testing 3 macro len's. A Nikon 60mm, a Tamron

90mm and a Sigma 105mm. All Nikon AF mounts. My question is

in regards to the f-stop reading on my N90s. As I focus the Nikon

or Tamron down to its closest focusing distance the f-stop reading

will change on my N90s...IE, f32 at infinity, f45 as you get around

1:2 and finally down to f64 at 1:1. I understand that the effective

f-stop is changing and allowing for the 2 stop light loss but the

Sigma doesnt do this. If I'm using aperture mode (as with the Nikon

and Tamron) and focus from infinity to 1:1 its f-stop never changes.

It reads f32 all the way thru its entire focus. Is this wrong? Maybe

Sigma doesnt feel they need to convey this info to the camera body.

Maybe they just state the actual mechanical aperture vs. effective

aperture. Any ideas? In case you are wondering my early (very early)

results are: Nikon has best build quality, Sigma is also very nice.

Tamron is all plastic but still well built. Optical results: This

may open a can of worms but so far all three are excellent. You

wouldnt be able to tell one from another. Just the early results...I

will post more as things develop. Thanks!!