What is a lighting ratio?

A lighting ratio compares the brightness of the lit side (key + fill) to the shadow side (fill only) of a face. A 2:1 ratio means the bright side is twice as bright as the shadow side (1 stop difference). A 4:1 ratio is 2 stops difference. Higher ratios create more dramatic, contrasty portraits.

What ratio should I use for portraits?

1:1 to 2:1 for beauty, fashion and corporate headshots. 3:1 is the classic portrait ratio — enough contrast to show form without losing shadow detail. 4:1 to 8:1 for dramatic, moody or character portraits. Higher ratios require careful shadow control and suit male or editorial subjects.

How do I set up a specific ratio?

Set your key light first for correct exposure. Then add fill light at reduced power. For a 3:1 ratio, the fill should be about 1.5 stops dimmer than the key. You can also control ratios by adjusting fill light distance (inverse square law) rather than power.

Does the ratio include ambient light?

In a studio with no ambient contribution, the ratio is determined entirely by key and fill lights. On location, ambient light acts as fill. To achieve higher ratios outdoors, you may need to overpower the ambient with your key light or use flags to block ambient from the shadow side.

PhotoCalcs

Key to Fill Ratio Calculator — Portrait Lighting Ratios

Calculate lighting ratios and contrast for portrait lighting, from flat beauty light to dramatic low key.

Lighting Ratio

Choose a preset or custom

Key Light Power

Key light power level

How We Calculate This

Lighting ratios describe the contrast between the bright and shadow sides of a subject:

Lighting Ratio

Ratio = (Key + Fill) / Fill

Stops Difference

Stops = log2(ratio)

Fill Power

Fill = Key power / (ratio - 1)

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard optical and photographic formulas. Results may vary with specific equipment.