Well, actually Viet, it depends upon how much ambient light there is. Sometimes when ambient light is lower, the flash will actually freeze the motion anyway, but you're right, you do need to be careful of that. When there is enough ambient light (see below) you'll get some blur, plus the flash 'stops' the action. In that case, ambient 'blur' adds to the image a little. Here's a father/daughter dance shot at 1/30th of a second at f4, ISO 400. There is certainly some ambient blur but the father/daughter are really sharp. But you get the sense of movement... see his foot. This is shot flash on camera bounced. (Probably 580 set to fire at about -1/3 or -2/3 flash exposure - ETTL)
HOSTED PHOTO
please log in to view hosted photos in full size.