can someone help explain what effective F-number variation is? I bought a new Nikkor 105mm f/2.8 micro lens today. By it's description I would expect that it would be able to reach an aperture of 2.8 at any focal length. After taking it for a test drive I've found that aperture closes down to f/3.5 at the shorter focal lengths. Is this correct or do I have a dodgy lens?