I recently got a powershot A85 and will be using it at an upcoming marathon. I will be a spectator and wish to photograph runners coming towards me. What mode should I use? Also, should I use continuous shooting? Also, after i push the button halfway down and wait for the camera to focus, how long do I have before I have to push the button all the way down. I know I am asking a lot of questions, but I am completely new to digital cameras and my son is running in this marathon so of course I want to take very good pictures. Sure would appreciate any help. Thanks.