Studio Lighting Ratio Calculator

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:

1. Meter key light + fill together → this is your “key side” reading
2. Meter fill light only → this is your “fill side” reading
3. 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.

Without a meter (speedlights): Set the ratio with distance or with power. Halving the power drops the light one stop, which is the easy lever. Distance follows the inverse square law, so moving the fill back to 1.4 times its distance also costs a stop, and doubling the distance costs two. Between those two you can hit any ratio on this page without metering anything.

Recommended ratios by genre:

• Newborn and baby work: 2:1, soft and gentle
• Corporate headshot: 3:1, professional and clean
• Beauty and glamour: 2:1 to 3:1
• Dramatic portrait: 4:1 to 6:1
• Film noir style: 8:1 or higher

Which convention are you using?

The dropdown asks this because the same lighting setup honestly has two ratio numbers and photographers use both. Compare the two lights and a 200 Ws key against a 100 Ws fill is 2:1. Compare what a meter reads off each side of the face and the same setup is 3:1, because the highlight side is getting key plus fill while the shadow side gets fill alone. Neither is wrong. Both are in print. The calculator shows you the other number underneath so you can quote whichever one your collaborator means.