Studio Lighting Ratio Calculator
Calculate studio lighting ratios between key and fill lights.
Find the exposure difference in stops and the resulting contrast on your subject.
A lighting ratio describes the difference in brightness between the key light (main light) and the fill light (shadow-fill light) falling on your subject.
The formula: Lighting Ratio = Key Light Power / Fill Light Power
When expressed in stops, each stop is a doubling or halving of light: Stops = log₂(Ratio) = ln(Ratio) / ln(2)
Common lighting ratios and their effect:
- 1:1: Flat, even lighting. No shadow. Good for high-key beauty shots.
- 2:1: Subtle shadow. Natural, flattering. One stop difference.
- 3:1: Classic portrait ratio. Slight shadow side. Industry standard for headshots.
- 4:1: Two stops. Dramatic but still detailed shadows. Good for character portraits.
- 6:1: Strong drama. Shadow side loses detail. Artistic/moody effect.
- 8:1: Three stops. Very dramatic. Shadow side near black. Fashion/editorial.
How to set a specific ratio: If your key light is at 200 Ws (watt-seconds) and you want a 3:1 ratio, set fill to 200/3 ≈ 67 Ws.
Measuring with a flash meter:
- Meter key light + fill together → this is your “key side” reading
- Meter fill light only → this is your “fill side” reading
- Take the difference in stops, call it d. Then key : fill = (2^d − 1) : 1
That subtraction matters, and it is subtle. Your meter sees the shadow side lit by fill alone, but the highlight side lit by key plus fill, because the fill light spills across the whole subject. So 2^d gives you (key + fill) ÷ fill, and you have to take the fill back out to get the key-to-fill ratio.
Worked through: if the lit side meters two stops brighter than the shadow side, d = 2, so 2² − 1 = 3, giving a 3:1 key-to-fill ratio. Skip the subtraction and you would call that same setup 4:1. Aim for 4:1 on a bad reading and you actually land on 3:1, which is about four tenths of a stop flatter than you wanted. Small, but it is the difference between a corporate headshot and a character portrait.
Guide number method (speedlights): If using speedlights without a meter, attenuate the fill light using distance (inverse square law) or manual power adjustments. Halving the power = 1 stop reduction.
Recommended ratios by genre:
- Newborn / baby: 2:1: soft and gentle
- Corporate headshot: 3:1: professional, clean
- Beauty / glamour: 2:1 to 3:1
- Dramatic portrait: 4:1 to 6:1
- Film noir style: 8:1 or higher
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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