Understanding multiple light studio portrait setups can be challenging. Here are some tips and hints to consider when using multiple lights in a portrait:
As a general rule, the exposure for your portraits should be based upon the main light, with fill light contributing little significant overall exposure. (An exception to this is "high key" lighting, where the fill and background lighting are comparable in subject lighting to the main light.)
Note that, even though I am going to be discussing strobes, the same effects can be obtained from photoflood lighting units, and even sunlight and reflectors. Just mentally substitute "flood" or "reflector" for "strobe".
Attractive portraits often require at least two light sources: one at the camera or to one side, and a second (often on the other side of the camera) for filling in shadows and making the lighting on the subject more even.
The distance or angle between the camera and the flash units is dictated by the light and shadow effect the photographer desires.
A very pleasing arrangement is to have the main flash to one side and above the camera, and to have the fill on the other side of the camera, and about one-half as far above the camera as the main light. A 30 to 60 degree angle between the flash units works well in most cases. Note that this effect can be done with lights with the same output by putting one closer to the subject than the other.
To analyze facial lighting in a portrait, look at the light/shadows on each side and under the nose. Then look at the shadows under the chin and cheeks. Finally, look at other areas to see if shadows show more information about the lighting setup.
Hair highlights and background shading will give you information on auxillary lights. The complete absence of shadows on the background usually indicates that one or more separate background lights were used.
Outstanding portraits often require three or more lights. Main, fill, highlight, and background. For some desired effects, more than one flash unit may be needed for some of these.
Look for and analyze other examples of multiple light portraits (or even advertising shots). Analyze the lighting setups used, then try some of your own.