Nikkor 105mm VR f2.8 micro not fixed aperture

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I have been playing with my new lens, and I have noticed that I an only get f2.8 when focussing at infinity, and when I focus closely, the camera reports f4.something. Surely then, it's a f2.8-f4.something lens, not a fixed aperture f2.8 lens as advertised.

What's going on? Is it something to do with it being on a DX body? Is it just some bad software getting it wrong? Is it really a variable aperture lens (on focus distance rather than the normal focal length you get in zooms)?

Don't get me wrong.... I am loving this lens.... It's the best one i own :-)
 
On Nikon bodies, it's the effective aperture rather than the actual aperture that is shown.

As you focus closer, less light reaches the sensor as if you have stopped the aperture down and it is this that gets reported rather than the actual aperture.
 
So that makes me think it's described incorrectly.

For example, the 18-55 kit lens with a variable aperture 3.5-5.6. When wide open, the diaphragm at the front of the lens is open by the same diameter, but different amount of light hits the sensor depending on where in the focal length/zoom range the lens is.

Surely, it's the same with this lens? The diaphragm remains fixed, by different amount of light falls on the sensor depending on where you are focussing to.

Seems a bit dodgy to me....
 
If you use almost any lens on the macro settings the aperture will effectively change, and at 1:1 you will get an effective aperture 1 stop lower than the nominal aperture - it's exactly the same as using extension tubes.

The f number is the aperture/focal length.

This doesn't alter the fact that the lens is f2.8 when used normally which is how all prime lenses are described since the aperture does not alter i.e the aperture remains fixed and so does the focal length.

Zoom lenses are described as (e.g) f2.8-f4 since their stop no. DOES alter as they are zoomed out.

.
 
Lens focal length and aperture are only true at infinity focus. At 1:1, you lose exactly two stops, ie f/2.8 lens becomes f/5.6 as far as exposure is concerned. Nikon reports this, Canon does not - they are both equally right and wrong technically speaking, but it's physics, not a lens fault.

Even more strange then, is why the Nikon 105 2.8 doesn't read f/5.6 at 1:1, but f/4.8. That's because the internal focusing mech also affects focal length and at 1:1 that lens is more like 80mm, but since the aperture remains the same physical size, that becomes f/2.4 at 80mm, dropping to f/4.8 at 1:1. If you follow :D

This happens a lot, especially with zooms, but passes unnoticed because TTL metering automatically compensates without us knowing, and we believe what it says on the lens barrel and Exif data. It's quite common for a 70-200mm zoom to be less than 150mm effective focal length at the long end when focused close.
 
That is a normal behaviour of a macro lens. Besides you don't really want a f2.8 when shooting 1:1, normally i m between f16-f22
 
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