I have a couple senior portrait shoots coming up in a couple of weeks. Since I don't have a studio, all the shots will be taken outside. I don't have a flash, other than the built in flash on my DRebel. If the weather is decent, I don't think I would need a flash, other than for some fill here and there. I will be using my 100mm f2.8 Canon lens as it is my sharpest lens, which means I will have a little distance between the camera and the girls. Will the on camera flash be strong enuf to be used as fill from say 10 or 12 feet away, or should I just suck it up and buy a real flash set up?