Quick Tip: A few days ago I posted about using Negative Fill in portraits. There were several questions about using negative fill, so I thought I would expand on the daily post. Fill light is achieved by using a light colored surface such as a white wall or reflector to bounce light at your subject. Negative fill is the exact opposite.
Placing the subject near a very dark object "reflects" a shadow. If you look at the boy in the driver's seat one side of his face is more brightly lit than the other. This was created by placing him near the dark frame of the car. Using negative fill in evenly lit situations creates more dimension with the play between light and shadow. Had his face been evenly lit, it would not have drawn your eye in.