[1] 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 (K) and fill (F).
The American Society of Cinematographers (ASC) defines lighting ratio as (key+fill):fill, or (key+Σfill):Σfill, where Σfill is the sum of all fill lights.
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.