I am shooting some outdoor tennis for the first time next week. I will be using a 7d and a 70 - 200 f2.8. I also plan to shoot manual for aperture, shutter speed and ISO. I also plan to use AI Servo. I have recently been using back button for focussing. Should I stick with it or would it be more sensible to use the shutter release button given that exposure is being dealt with manually?