When lighting a subject, the goal is usually to create a natural effect. Outdoors, the brightest (or main) light source is the sun. This implies that you would use one dominant light to illuminate the subject, and cast the major shadows. Shadows are lightened later, with supplemental lights or reflectors.

When positioning the main light, you should notice the placement of shadows as you raise the light or move it relative to the subject.

Look at the way the shadows move across the face when the main light is repositioned.