Yes, you're totally right - The 500mm is definitaly sharper wide open. Results become very similar when the 600mm is stopped down to f/5.6.


But, what I really think it comes down to is the focal length. I think that both lenses perform superbly, and those who need 500mm and f/4 will go with the 500mm, and those who need 600mm and f/4 will go for the 600mm. I don't think that the 500mm's optical performance is so impressive (relative to the 600mm) that it becomes the deal-breaker. It can definitaly sway your decision though.