Does anyone know how to lock the autofocus on centre. I am using a Sigma 150-500mm lens for bird watching. Yesterday I'm zoomed fully in with a bird in centre of the frame and nothing other than sand and water to the left, right, above and below. When I squeeze the trigger the green focus boxes are flashing on 2 points either side of the bird. Why does my camera think I want to focus on the water when I've obviously got the main subject in the middle of the frame? Am I missing something? There must be a way to switch off this function and focus solely on what is in the middle of the frame. Any ideas?