Help pick a portrait lens for the holiday

Hi Thierry,

You raised a very good point! My bottom line question is: How different is 90mm f/2.8 vs. 85mm/1.8?

When I was hunting, "Nikon 105mm f/2.8G ED-IF AF-S VR Micro-Nikkor Lens" was also within my radar. Among many good reviews for 105mm at Amazon, S. L. Miller commented that Nikon 105mm "still catching up to the Tamron 90", see:

http://www.amazon.com/review/R1ACFNNDABL4M5/ref=cm_cr_pr_viewpnt#R1ACFNNDABL4M5

I then took a quick on Tamron  90mm f/2.8, but I thought 90mm was about the same as 85mm, so I left out Tamron  90mm f/2.8 from my list.

Is Tamron 90mm f/2.8 better than Nikkor 85mm/1.8?

Further, why f/2.8 is always more expensive than f/1.8?