I was shooting a kayak race with the Nikon D3 & Nikkor 200-400mm f/4G ED-IF AF-S VR. Between runs I noticed a pretty girl in the gallery and without giving it much thought I turned and snapped a candid shot. While editing the shots later I was impressed by the fine detail captured in the photo. See the crop here http://www.pbase.com/jboard/image/114970852/original It's not a controlled test or scientific lens comparison, just a "wow!"

I'd have never thought of a 200-400 zoom as a portrait lens but it did a pretty fair job on this grab shot. :)