In a two-light set up the key light is
generally placed above the camera and at about 45°
angle to the subject. For flatter, less directional lighting, you
might want to place it above and directly behind you camera, with a
diffusing filter in front of the light.
The fill light is placed opposite the key light
to soften the shadows. Either the positioning of the light or the relative
wattage of the two lights can be used to make the intensity of your fill
light less than that of your key light.
For portraiture, you may
want to use one or both of your lights "bouncing" from an umbrella.
This creates a soft, diffused light which cast fewer unflattering shadows
than direct lighting.
Your third light can be uses in a number
of ways. In portraiture, it can be directed at the subjects hair to
add sparkle. Make the intensity of the hair light slightly grater than
that of your key light.
It can also be uses as a
background light, to separate subject from the background. Try this
effect when shooting on a small set, to add the illusion of more space.
Aimed directly on the background, you can brighten
your shot, add separation between the subject and the background, or use
color effects gels on your background light to create any color background
you like.