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Lighting Ratio Calculator

Enter key and fill light levels to find the lighting ratio and stop difference.

Result

5.00:1
2.00 stops
4.00:1

How it is calculated

Enter the light level of the key light and of the fill light in the same unit (lux, foot-candles, or any consistent meter reading), each measured on its own at the subject. The tool returns three numbers. The classic lighting ratio is (key + fill) ÷ fill, which counts the fill light as part of the lit side because on the lit side of a face both lights add up. The plain key-to-fill ratio is key ÷ fill. The difference in stops is log₂(key ÷ fill). Both levels must be above zero. Because each stop doubles the light, the stop value is negative if the fill is brighter than the key. Results show two decimals. The tool measures only the ratio of light levels and says nothing about exposure itself.

Worked example

With only the key light on, a meter reads 800 lux at the subject's face. With only the fill light on, it reads 200 lux. The key-to-fill ratio is 800 ÷ 200 = 4.00:1. The classic lighting ratio adds the fill to the lit side: (800 + 200) ÷ 200 = 5.00:1. The stop difference is log₂(4) = 2.00 stops, since 200 doubled twice gives 800. The classic ratio is therefore one step above the key-to-fill ratio in this example, which is why quoted ratios in tutorials and tools sometimes disagree.

Try this example →

Common mistakes

  • Measuring the key light with the fill light already switched on and entering that reading as key; the classic ratio assumes each light is measured alone.
  • Mixing readings in different units or taken at different distances from the subject.
  • Comparing the classic ratio (key + fill to fill) with a stop difference: a 5:1 classic ratio is 4:1 key-to-fill, which is 2 stops, not about 2.3 stops.
  • Entering fill as 0 to mean no fill light; the ratio would be infinite, so the tool returns no result. Use a small reading instead.
  • Entering the two lights in reversed order and getting a negative stop value or a key-to-fill ratio below 1:1.

About this calculator

In photographic and cinematic lighting, the lighting ratio compares how much brighter the key light is than the fill light, expressed in stops — each stop is a doubling of light, so the ratio is calculated as stops = log₂(key light level ÷ fill light level). A 1-stop difference is a 2:1 key-to-fill ratio, which the classic lighting ratio (key + fill) : fill shown as the headline result reports as 3:1 (mild, flattering contrast); 2 stops is 4:1 key-to-fill, or 5:1 classic (the dramatic portrait look), and 3+ stops pushes into deep, moody chiaroscuro territory.

Portrait and studio photographers use this to dial in consistent, repeatable looks across a shoot, while cinematographers and gaffers use the same stop math when setting up key-to-fill contrast for a scene's mood, matching it to the story's tone or a director's shot list.

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Comments (1)

  • Editorial team

    Moving a light is the quickest way to change the ratio, because a small source loses a quarter of its level each time its distance doubles. Start from the example of 800 lux key and 200 lux fill. Doubling the fill's distance leaves 50 lux, so the key-to-fill ratio goes from 4:1 to 16:1, that is 2 stops to 4 stops. Read the fill with a meter after each move. How do you usually set your fill, by distance, power or flags?

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