b. F/Stop Distance Scale Method. Just as f/8 puts twice as much light on the film as does f/11,
a light positioned at 8 feet puts twice as much light on the subject as when it is positioned at 11 feet.
Example: The fill light should produce no shadows on the subject, and when positioned at 11 feet will
put one unit of light over the entire subject.
The main light creates shadows and when positioned at 8 feet will put twice the amount of light on the
area of the subject it illuminates. The shadows have one unit of light from the fill, while the highlighted
areas have one unit from the fill and two units from the main. This produces a 3:1 ratio.
If the main light was moved to 5.6 feet it would be putting four times more light on the subject than the
fill light at 11 feet. Therefore, the main plus the fill as compared to the fill alone would give the
photographer a 5:1 ratio.
c. Power Setting Method. To establish a 3:1 lighting ratio you can also establish both the main
and the fill light at equal distances from the subject, but cut the strength of the fill by one-half. This can
be done through power settings or by using a diffusing screen over the fill light.
d. Flash Meter Method. Use a flash meter for optimum exposure and lighting ratio control.
Guide numbers are adequate, but for more precise measurements you should use a flash meter. A flash
meter will also enable you to establish your own guide number for each flash unit.
Flash meters measure the incident light coming from the flash units. You use a flash meter by placing it
at the subject position and pointing it at the camera. Trip the flash and you get an immediate and
accurate flash exposure reading. One of the greatest advantages of a flash meter is that you can easily
compute exposure if you are using more than one flash unit or if you are using bounce flash. Take a
flash meter reading at the subject, of the main alone, and then the fill alone. This way you can calculate
your ratio.
Example: Using an incident flash meter at the subject position, adjust the fill light so it is not metered.
With the main light at the desired position and distance from the subject, take a flash reading of the main
light alone (f/11 is read). Adjust the main light so it is not metered. Adjust the fill light and take a
reading (f/8 is read). Since the main light is putting twice the amount of light on the subject as is the fill,
the ratio is 3:1 (ratio = main light + fill light: fill alone).
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