Free Film & Video Tool · Calculator
Lighting Ratio Calculator
Convert lighting ratios to stops and back using the lit-side to shadow-side convention, and see how that differs from a key-to-fill ratio metered light by light.
Deterministic maths that runs entirely in your browser — no AI model, no uploads, no account. Formulas and assumptions are explained below the calculator.
Inputs
Results update as you type.
Type 4 or 4:1.
Results
Lit : shadow ratio
4:1
2 stops difference
Shadow side
25%
of the lit side's light
Key-to-fill, metered light by light
3:1 (1.58 stops) — the key alone reads this much brighter than the fill alone.
Assumes the fill lights both sides equally, the key lights only the lit side, and light adds. That's why a key 1 stop over the fill gives 3:1 lit-to-shadow, not 2:1. The two numbers measure different things and aren't interchangeable.
How it works
A lighting ratio describes contrast across a subject. People use the term for two different measurements, so this calculator states its convention and converts between the two under clearly stated assumptions.
The convention used
Lighting ratio = lit-side illuminance ÷ shadow-side illuminance, as measured with an incident meter on each side of the face with all lights on. Stops = log₂(ratio).
| Lit : shadow | Stops | Shadow side gets | Key : fill (metered separately) |
|---|---|---|---|
| 1:1 | 0 | 100% | — (flat) |
| 1.5:1 | 0.58 | 66.7% | 0.5:1 |
| 2:1 | 1 | 50% | 1:1 |
| 3:1 | 1.58 | 33.3% | 2:1 |
| 4:1 | 2 | 25% | 3:1 |
| 8:1 | 3 | 12.5% | 7:1 |
| 16:1 | 4 | 6.3% | 15:1 |
Key-to-fill is a different measurement
If you meter the key light with the fill switched off, and the fill with the key switched off, you get a key-to-fill ratio. To convert, the calculator assumes the fill reaches both sides of the face equally, the key reaches only the lit side, and light adds:
- lit side = key + fill; shadow side = fill
- lit : shadow = (key ÷ fill) + 1, so key ÷ fill = (lit : shadow) − 1
That's why a key 1 stop over the fill (2:1 key-to-fill) produces a 3:1 lit-to-shadow ratio. If your fill is off-axis, your key wraps around the face, or there are other sources, the conversion won't hold — measure the lit and shadow sides directly.
Limitations
- The swatch is illustrative: screens, skin tone, camera gamma and grading all change how a ratio looks on screen.
- Practical lights, bounce and the background also shape the perceived contrast.
- Planning the setup itself? The lighting setup planner gives a written key, fill and backlight plan.
FAQ
- How many stops is a 4:1 lighting ratio?
- Two stops, using the lit-side to shadow-side convention: each stop doubles the light, and 2 × 2 = 4.
- Is a key light 1 stop over the fill a 2:1 ratio?
- Not on the subject. If you meter the key alone and the fill alone, a 1-stop difference is a 2:1 key-to-fill ratio. But the lit side receives key plus fill, so the lit side is 3 parts to the shadow side's 1 — a 3:1 lit-to-shadow ratio (about 1.6 stops).
- Which ratio should I use?
- There's no single right answer — low ratios (around 2:1) read as soft and even, high ratios (8:1 and above) as dramatic and low-key. Decide by look, then use the numbers to repeat it consistently across setups.
- Is it free to use?
- Yes. There's no signup or account, and the calculations run entirely in your browser — nothing you type is sent to a server or an AI model.
- Can I share or save my results?
- Yes. Your inputs are saved in the page address as you type, so “Copy link” gives you a URL that reopens the calculator with the same settings. “Copy results” copies a plain-text summary.
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