I purchased a D4S last week and have been testing it. I tried shooting some BIF against a blue sky. Before the auto focus could grab the bird it racked out to infinity and stayed in that position until I released the auto focus and re-focused on a new subject. I tried it again on an odd shaped cloud with the same result. This morning I tried to some seagulls in flight against am overcast sky with the same issue. This has happened with 3 different lenses, 200-400, Sigma 120-300 and Nikon 70-300. Focus and meter modes have no affect, same issue. I do not have the problem with wither the D3S or D810. If I point either one at a clear sky the auto focus will just hunt until something enters the focus area. The D4S just "locks" up at infinity. Is this just peculiar to the D4S or (as I suspect) a defective auto focus system?