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 — the additive convention used by the ASC and studio meters. Because the bright side is lit by the key AND the fill while the shadow side has fill only, the stops between the two lights is log2(ratio − 1): a 2:1 ratio means the key equals the fill (0 stops, flat), a 3:1 ratio means the key is 1 stop brighter than the fill, 5:1 is 2 stops and 9:1 is 3 stops. Higher ratios create more dramatic, contrasty portraits.

What ratio should I use for portraits?

2:1 (flat, key = fill) for beauty, fashion and high-key headshots. 3:1 is the classic portrait ratio — the key 1 stop over the fill gives enough contrast to show form without losing shadow detail. 5:1 to 9:1 (2–3 stops) 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 exactly 1 stop dimmer than the key (half the key power); for 5:1 set the fill 2 stops down. You can also control ratios by moving the fill light further away (inverse square law) instead of dialling its 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 RatioPick a (key+fill):fill ratio preset, or Custom to enter stops directly

Key Light Power

Your key light's power, as a percentage of full output

How We Calculate This

Lighting ratios describe the contrast between the bright and shadow sides of a subject. This calculator uses the additive convention: the highlight side is lit by the key and the fill together, while the shadow side receives the fill alone.

Lighting Ratio

Ratio = (Key + Fill) ÷ Fill

Stops Difference (key over fill)

Stops = log₂(Ratio − 1) — so 2:1 = 0 stops (key = fill), 3:1 = 1 stop, 5:1 = 2 stops, 9:1 = 3 stops.

Fill Power

Fill = Key power ÷ (Ratio − 1) — at 3:1 the fill is half the key (1 stop down).

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.