I work in a ballroom dance studio and take photos of dancers at showcases and exhibitions. Our floor is 30 x 60 feet, and I sit about in the middle of the long side and the dancers are all over the floor. Lighting is medium to low. I seem to be having trouble with focus. Should I be using aperture or shutter? Sometimes the feet are blurry as I am not shooting with a fast enough shutter? Any suggestions appreciated.