First, the color is more intense. The main light strikes the subject at an angle, sending the reflected, colorless light off axis, away from the lens. Except for highlights, the absorbed and scattered light entering the lens contains only saturated color. The light on the camera serves two purposes: to keep the density level of the shadow areas high enough for good contrast and detail, secondly to evenly light the whole picture area, and thirdly to flatter the faces and remove the bags under the eyes.
In order to achieve the above benefits, a proper ratio must be retained. One easy way is to set the lens on F8, the fill flash on F8 automatic, and the modeling light on F11 automatic. This configuration should produce twice as powerful a light coming from the modeling light as from the fill light no matter what the distance from the lens to the subject. Ideally, a flash meter should be used to verify and fine tune the output. If a
flash meter is not available, a few test exposures will suffice.
A second method sets the fill flash on manual one half power and the modeling light at manual full power. This setting requires the F stop to be changed if the distance from the lens to the subject changes significantly unless OTF (off-the-film) is available in the camera.
While the fill flash can be conveniently kept small, there are several options available for the modeling light. A small silver umbrella reflector (minimum 18 inches in diameter) is the least expensive and does a good job. Better is a soft box (minimum 12 inches in diameter) but requires a more powerful head. Simplest is bare bulb which is soft but not too flattering to faces and requires a lot of power. Four hundred ISO is fast enough to use with 100 WS electronic flash. I recommend the use of a radio slave to trigger the second light for a guaranteed proprietary use. Other flashes in the room will have no effect on your second light, saving the batteries and preventing other photographers from using your light and/or ruining their own pictures.
Though a light stand is sufficient to support your second light, with a little instruction, a live person will save you a lot of running around. The modeling light may be situated to the photographer’s right or left side, producing a thirty degree angle to the subject. If the subject is twenty feet away from the lens, the modeling light should be seven feet to your right or left. An added benefit is that the backgrounds will be softly lit, but will never overpower the subject in importance.
For those photographers working with a digital camera with a built in pop up flash, a small slave flash will do the job. Vivitar makes a slave flash with a guide number of 93.
This is three times more powerful than the flash in the camera but 1/4 or 1/8 power can be set for a better ratio. With the camera ISO set on 200, the Vivitar DF200 is powerful enough to be used as a bounce flash. Aim the slave flash into a corner of the room (half on the ceiling) behind you and to the side. Use full power on the flash and try setting the aperture on F5.6. The camera flash may need to be softened with a piece of translucent plastic. I recommend a few test exposures to verify the ratio. One benefit of bounce flash is the nice modeling you get on the faces. Another is that the same camera settings can be used for any distance from the camera. You will be amazed at the saturated colors and the three dimensional effect of the modeling on the subject.