i finally got some time to play around with my new 400 5.6 but of course the wind was blowing a steady 30mph with gusts up to 50 again today . i decided to play around a little in the house with it and got the afma pretty close to where it needs to be at +4 then i set up a model to compare the 2 lenses. i thought i was getting pretty decent detail in my bird pics with the zoom lens but i can clearly see more detail with the prime when i zoom in on the image . i also noticed there isn't much of an improvement stopping down to f8 with the prime but a very noticeable difference with the zoom stopped down. here are 2 heavily cropped images where you can see the increased detail. its not a huge difference but its better. now if the weather would cooperate a little i might be able to find some critters to try it on .
prime wide open
400 f5.6 @ 5.6 by sedwards679, on Flickr

100-400 @ f8
100-400 @ f8 by sedwards679, on Flickr