What is lighting ratio, and how do you calculate it?

Asked 8/26/2010

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In photography lighting, what does a lighting ratio mean between the main light and fill light? How is it calculated, and how does it affect contrast in the final image?

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Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0

Photography Stack Exchange contributor

16y ago

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It's the difference between your main light and fill light expressed as a ratio of brightness. Example: if your main light is twice as bright as your fill light, the ratio is 2:1. A high ratio will give a more contrasty effect, as shadows cast by the main light will still be dark. A lower ratio means the fill light lessens the effect of shadows cast by the main light, giving a flatter effect.

Note a difference of one f-stop is a factor of two, so a ratio of 2:1. Similarly, two stops is 4:1, and three is 8:1.

Originally by user456. Source · Licensed CC BY-SA 4.0

user456

16y ago

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Lighting ratio describes the brightness relationship between your main light and fill light, usually written as a ratio such as 2:1 or 4:1.

In simple terms, the stronger the main light is compared with the fill, the higher the ratio and the more contrasty the image will look. Higher ratios leave shadows darker; lower ratios reduce shadow depth for a flatter, softer look.

A useful rule is that each 1-stop difference equals a doubling of light:

1 stop difference = 2:1
2 stops difference = 4:1
3 stops difference = 8:1

So if your main light is twice as bright as your fill light, that’s a 2:1 ratio. If it’s four times as bright, that’s 4:1.

Practically, low lighting ratios give more even lighting, while high ratios create stronger modeling and drama.

What is lighting ratio, and how do you calculate it?

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