Lighting ratio
Lighting ratio in photography refers to the comparison of key light to the total fill light. The higher the lighting ratio, the higher the contrast of the image; the lower the ratio, the lower the contrast. The lighting ratio is the ratio of the light levels on the brightest-lit to the least-lit parts of the subject; the brightest-lit areas are lit by both key and fill. The American Society of Cinematographers defines lighting ratio as :fill, or :Σfill, where Σfill is the sum of all fill lights.
Light can be measured in footcandles. A key light of 200 footcandles and fill light of 100 footcandles have a 3:1 ratio — :100.
A key light of 800 footcandles and a fill light of 200 footcandles has a ratio of 5:1 according to the lighting ratio formula — :200 = 1000 / 200 = 5 : 1.
The ratio can be determined in relation to F stops since each increase in f-stop is equal to double the amount of light: 2 to the power of the difference in f stops is equal to the first factor in the ratio. For example, a difference in two f-stops between key and fill is 2 squared, or 4:1 ratio. A difference in 3 stops is 2 cubed, or an 8:1 ratio. No difference is equal to 2 to the power of 0, for a 1:1 ratio.