Inverse Square Law:
The inverse square law (law of squares) tells us how to calculate the distance and placement of a lighthouse. This can also help determine falloff, or how dark the shadows in the background of the picture will become according to F-stops. To put it simply you can achieve the same exposure for your subject, by having a smaller light closer and a bigger light placed farther away. Obviously though, the smaller light will not make the larger space in the background as bright as the larger light which is farther away will. We know this because you can take the illuminated space of a light ten feet in distance is a quarter of what it was at twice the distance. At a third of the distance, it is a quarter of what it was at ten feet. Taking advantage of this law and learning how to manipulate light sources with available equipment are serious things to consider in how to achieve the look for the right situation.
Harry Box's Set Lighting Technicians Handbook
I highly recommend anybody interested in film lighting to pick up this book, it is very well written and provides an incredible amount of useful information about film and video lighting.