Here's another excellent video by Mark Wallace. In this video Mark talks to you about Lighting Ratios,
positioning of your key and fill lights
and the importance of having consistent lighting for obvious reasons. Positioning of your lights plays an important role on the contrast of the image.
1:1 Ratio - Same Output
2:1 Ratio - 1 Stop difference
4:1 Ratio -2 Stop difference
8:1 Ratio - 3 Stop difference