It's most likely when you have your camera at the 500mm focal length (really zoomed in) you'll be able to use the macro feature of your lens/camera. This should enable you to focus on things that are closer rather than farther away. There is probably a switch on the barrel of your lens that would activate this feature.
"To photograph is to hold one's breath, when all faculties converge to capture fleeting reality. It's at that precise moment that mastering an image becomes a great physical and intellectual joy."
~ Henri Cartier-Bresson