Lighting Ratio Calculator
Lighting ratio describes the contrast between the bright (key-lit) side and the shadow (fill-lit) side of a subject. It is calculated as (key + fill) : fill, because both lights illuminate the key side but only the fill illuminates the shadow side. A 4:1 ratio means the highlight side receives four times as much light as the shadow side, giving about 2 stops of contrast. Enter your key and fill light intensities as relative values (power settings, watt-seconds, or incident meter f-stop numbers) to find the ratio and stop difference.
Lighting ratio formula
Lighting ratio = (key + fill) / fill
Stop difference = log2(ratio)
Example: key = 3, fill = 1. Ratio = (3+1)/1 = 4:1. Stop difference = log2(4) = 2 stops. The key side is 4 times brighter than the fill side, which is a 2-stop difference. Common for portrait photography.
Lighting ratio reference guide
- 2:1 ratio (1 stop): flat, even, suitable for beauty and fashion
- 3:1 ratio (1.58 stops): flattering for most portrait subjects
- 4:1 ratio (2 stops): standard portrait, clear facial modeling
- 6:1 ratio (2.58 stops): dramatic, beginning to lose shadow detail
- 8:1 ratio (3 stops): theatrical, deep shadows, strong contrast
- 16:1 ratio (4 stops): extreme chiaroscuro, used for artistic effects
Lighting ratio calculator: frequently asked questions
What is a lighting ratio in photography?
The lighting ratio is the comparison of the intensity of the key (main) light to the fill light on the shadow side of the subject. A ratio of 2:1 means the key side is twice as bright as the fill side. A 4:1 ratio gives stronger shadows and more dramatic contrast.
How is lighting ratio calculated?
The lighting ratio is the sum of light on the key side divided by the fill alone. Because the fill light also illuminates the key side: Lighting ratio = (key + fill) / fill. If key = 3 and fill = 1 (both in relative units or incident meter f-stops), ratio = (3+1)/1 = 4:1.
What f-stop difference corresponds to common lighting ratios?
Stop difference = log2(ratio). A 2:1 ratio is 1 stop. A 4:1 ratio is 2 stops. An 8:1 ratio is 3 stops. A 16:1 ratio is 4 stops. Practical studio work commonly uses 2:1 for flat, even light, 4:1 for standard portrait, and 8:1 for dramatic or theatrical looks.
How do I measure lighting ratio in practice?
Use an incident light meter. Meter with only the key light: record the f-stop. Meter with only the fill light: record that f-stop. The stop difference tells you the ratio. Alternatively, use the power settings on flash units: a 2-stop difference in power gives a 4:1 ratio.
What lighting ratio is most flattering for portraits?
A 3:1 or 4:1 ratio (about 1.5 to 2 stops) is widely considered most flattering for portraits, providing enough shadow to give facial dimension while keeping detail in both the highlight and shadow sides. A 2:1 ratio is very flat and suitable for commercial or beauty work.
Official sources
- ISO 12232:2019: Photography, Digital still cameras, Determination of exposure index. ISO.org.
- CIE 015:2018: Colorimetry, light measurement standards. CIE.co.at.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.