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i own the 500 f/4, and i had the 100-400 and 400 f/5,6 in the past.
Well, i can tell you this. The 100-400 is a quite good lens, and can give really good images, especially on 'strong' body (on my 5Dii was great).
But shooting with a 500 f/4, or a 400 f/2,8 (i took some shots with that one) - i never used the 400DO- is an other class. Images are sharper of course, and the use of that f/4 is really great, producing that great blurry backround.
What i noticed with my 100-400 (and the same thing one friend of mine that owns the 100-400) is that when we were using the f/5,6 aperture, the in focus images were reducing to a great degree..At 6,3 you could get a better rate of in focus images and at 7,1 i believe we had the best results (sharper images as well as in focus images rate). What i mean is that with the 100-400 at 5,6 it's somehow a risk to shoot, because the chance of geting out of focus image gets higher. At least that's my personal experience, which is also supported from that friend of mine.
But in general, is a very versatile and nice lens.
On the other hand, shooting at f/4 is a satisfaction..If my speed isn't too low of course, it produces very sharp and in focus images, much better than the 100-400.
Hoped i helped a little,