I have a canon rebel xti and I am purchasing a Canon Speedlite 580EX II. I will use the camera for indoor gymnastics with poor lighting in the building, indoor dance recitals with poor lighting and indoor or outdoor events with family and kids. Should I purchase a diffuser? This may be a silly question, but I am not sure what it does. If it would provide a softer light I may need one. Could someone please tell me what a diffuser is used for and what one they would recommend? Any books anyone would recommend on flash photography? Thank you.