Studio Lighting Ratio Calculator
| Ratio | F-stops | Shadow % | Look | Best For |
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The lighting ratio compares the brightness of the key light, which is the main light, with the fill light that softens the shadows. A low ratio such as 2 to 1 gives soft, even light with gentle shadows, and a high ratio such as 8 to 1 gives deep shadows and a dramatic, moody look. Many portrait photographers use ratios between 2 to 1 and 4 to 1 for flattering skin tones.
Each stop of difference doubles the light, so a difference of one stop between the key plus fill and the fill alone gives a 2 to 1 ratio, and two stops gives 4 to 1. The calculator also works from the power of the two lights. Conventions differ, since some people state the ratio of key to fill and others compare the combined light to the fill, so check which version you are using.
To change the ratio, move the fill light, change its power, or add a reflector. Because light falls off with the square of the distance, moving a light back by about 1.4 times its distance cuts its effect by one stop. Use a light meter to measure each light separately, and adjust until the ratio suits the mood you want.