Lighting Ratio Calculator
Key-to-fill ratios in stops — where a "2:1 lighting ratio" turns out to mean both lights are set to exactly the same power.
A ratio describes brightness and nothing else — not the quality of the light, not its direction, not where the shadow edge falls. A 4:1 from a bare bulb and a 4:1 from a large softbox measure the same and look nothing alike. It also assumes the fill reaches both sides evenly, which a fill placed close to the camera roughly does and one placed off to the side does not. Nothing is uploaded.
A "2:1 ratio" means both lights are set to the same power
The quoted ratio is the ratio of the two sides of the face, because that is what a meter reads. But the key side receives the key and the fill, while the shadow side receives the fill alone — so a side ratio of R comes from lights set at R − 1 to 1. At 2:1 that is 1:1: two lights, identical power. A 4:1 is lights at 3:1, and only far out at 16:1 do the two readings converge, by which point the shadow is four stops down and nobody is arguing.
| Ratio quoted | Side difference | Lights actually set at | Which is | Gap |
|---|---|---|---|---|
| 1:1 | 0.000 stops | no fill at all | 0.000 stops | 0.000 |
| 2:1 | 1.000 stops | 1:1 | 0.000 stops | 1.000 |
| 3:1 | 1.585 stops | 2:1 | 1.000 stops | 0.585 |
| 4:1 | 2.000 stops | 3:1 | 1.585 stops | 0.415 |
| 6:1 | 2.585 stops | 5:1 | 2.322 stops | 0.263 |
| 8:1 | 3.000 stops | 7:1 | 2.807 stops | 0.193 |
| 16:1 | 4.000 stops | 15:1 | 3.907 stops | 0.093 |
The gap between the two readings is widest exactly where portrait photographers work — a full stop at 2:1, 0.415 at 4:1 — and narrows to 0.093 only by 16:1. If you set your lights from the quoted number without allowing for it, every portrait comes out flatter than you intended.
Setting the lights first, and the looks each ratio gives
Going the other way is the same arithmetic backwards: lights at K:1 produce a side ratio of K + 1. Equal lights give 2:1, a key three times the fill gives 4:1. The ratio itself is a base-two log, so the powers of two are whole stops and a 3:1 is 1.585 — which is why the classic series runs 2, 4, 8, 16 rather than 2, 3, 4, 5.
| Lights set at | Meter will read | Side difference |
|---|---|---|
| 1:1 | 2:1 | 1.000 stops |
| 2:1 | 3:1 | 1.585 stops |
| 3:1 | 4:1 | 2.000 stops |
| 4:1 | 5:1 | 2.322 stops |
| 7:1 | 8:1 | 3.000 stops |
| 15:1 | 16:1 | 4.000 stops |
And what each ratio actually looks like. The standard portrait ratio is under two stops, with its two lights only one stop apart — a much gentler setup than the numbers suggest.
| Look | Ratio | Stops |
|---|---|---|
| Flat, no modelling | 1:1 | 0.000 |
| Gentle, high key | 2:1 | 1.000 |
| Standard portrait | 3:1 | 1.585 |
| Defined, classic | 4:1 | 2.000 |
| Dramatic | 8:1 | 3.000 |
| Near silhouette | 16:1 | 4.000 |
How to use
- Enter the lighting ratio you want, as R:1.
- Read the stop difference and what the lights must be set to.
- Or enter your key and fill powers to get the ratio.
- Compare the named looks in the table.
Frequently asked questions
What does a 2:1 lighting ratio mean?
That the lit side of the face is twice as bright as the shadow side — one stop. What surprises people is what it takes to get there: two lights set to exactly the SAME power.
Why are the lights equal at 2:1?
Because the key side receives the key AND the fill, while the shadow side receives the fill alone. A side ratio of R comes from lights set at R minus one to one, so at R = 2 the lights are 1:1.
So what are the lights at 4:1?
Three to one, which is 1.585 stops apart rather than the two stops the ratio names. The quoted number is always the larger of the two, and always describes the faces rather than the fixtures.
How big is the difference between the two readings?
Widest exactly where portrait photographers work: a full stop at 2:1, 0.415 at 4:1, and under a tenth only by 16:1 — by which point the shadow is four stops down and nobody is arguing about it.
How many stops is a 3:1 ratio?
1.585, because the ratio is a base-two log. Only the powers of two land on whole stops, which is why the classic series runs 2, 4, 8 and 16 rather than 2, 3, 4, 5.
What ratio should I use for a portrait?
Around 3:1 is the conventional starting point — under two stops between the sides, with the two lights only one stop apart. 2:1 is gentle and high key, 8:1 is dramatic, and 16:1 is close to a silhouette.
Does the ratio describe the look?
Only the brightness part of it. A 4:1 from a bare bulb and a 4:1 from a large softbox measure identically and look nothing alike, because the ratio says nothing about the quality or direction of the light.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.