Studio Lighting Ratio Calculator — Key, Fill & Contrast in Stops
Enter lighting measurements
Calculated contrast
Classic combined convention: (key + fill) ÷ fill. The separate key source is 2 times the fill, producing a 3:1 lit-to-shadow ratio.
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How to measure key and fill consistently
- Choose a method. For isolated sources, switch off or fully flag the fill while reading the key, then isolate the fill and ambient baseline. For side readings, measure the key-lit side and the corresponding fill-only shadow side.
- Keep the setup fixed. Use the same meter, unit, position, orientation, diffuser, and subject reference for both readings. Do not change fixture output between them.
- Use linear measurements. Enter lux, foot-candles, or proportional linear values. Do not enter f-numbers such as 5.6 and 2.8; aperture numbers do not scale linearly with light.
- Read both outputs. “Key-only : fill-only” compares sources. “Lit-side : shadow-side” compares total illumination and follows the classic combined lighting-ratio convention.
- Judge the recorded image. Confirm the intended mood, skin tone, highlight retention, and shadow detail on the actual camera or capture medium.
Metering note: reflected readings also include subject reflectance. When comparing two sides with a spot meter, use equivalent-tone areas or the same neutral target so reflectance does not masquerade as a lighting difference.
Key, fill, ratio, and stop formulas
Let K be the isolated key contribution and F the fill-plus-ambient baseline. The key-lit side receives both contributions, while the shadow side receives the baseline:
lit side = K + F
shadow side = F
combined lighting ratio = (K + F) ÷ F
The source ratio and each stop difference are:
key-to-fill source ratio = K ÷ F
combined contrast stops = log₂((K + F) ÷ F)
key-versus-fill stops = log₂(K ÷ F)
Example: a key-only reading of 200 lux and fill-only reading of 100 lux gives a 2:1 source ratio. Together, the lit side receives 300 lux while the shadow side receives 100 lux, giving a 3:1 combined ratio and log₂(3) ≈ 1.58 stops of combined contrast.
References: Kodak defines lighting ratio as key plus fill compared with fill alone in its Glossary of Motion Picture Terms. The Defense Information School studio lighting guide explains how higher ratios increase lit-to-shadow contrast. Terminology varies, so this calculator always labels combined and source ratios separately.
Target studio lighting ratios
The table uses your current fill or shadow-side reading as the baseline. “Required key-only” is the added directional key contribution needed under the combined convention.
| Combined ratio | Combined contrast | Required key : fill | Required key-only reading |
|---|
Assumptions and practical limits
This is an additive intensity model. It assumes both readings use the same linear scale and that the fill or ambient contribution reaches both measured sides comparably. It does not model light direction, source size, modifier transmission, colour, subject reflectance, specular response, lens flare, camera tone curves, film response, exposure, or grading.
A numeric ratio is a repeatable setup reference, not a universal recipe for a particular mood. Different faces, materials, camera systems, and display transforms can look different at the same measured ratio. Use the result for planning and continuity, then verify with a meter and test image.
Studio lighting ratio FAQ
What is a studio lighting ratio?
In the classic combined convention, it is the light on the key-lit side—key plus fill—compared with fill alone on the shadow side. Because “key-to-fill” is also used for the direct source comparison, this page reports both values.
How do I convert a lighting ratio to stops?
Take log₂ of the linear lit-to-shadow ratio. Ratios of 2:1, 4:1, and 8:1 correspond to one, two, and three stops of combined contrast.
Is a 3:1 lighting ratio the same as key being three times the fill?
No under the combined convention. A 3:1 lit-to-shadow ratio requires the isolated key to be twice the fill: (2 + 1) ÷ 1 = 3. A direct 3:1 key-to-fill source ratio would produce a 4:1 combined ratio.
Can I enter f-numbers or T-stops as readings?
Do not enter the numeric aperture values directly because they are not linear intensities. Take linear lux or foot-candle readings, or convert an already known stop difference to a linear ratio with ratio = 2 raised to the number of stops.
Does ambient light count as fill?
Yes when it forms part of the shadow-side baseline. Include ambient and bounce with the fill reading, but avoid counting the same ambient contribution again inside an isolated key-only reading.
What does a 1:1 combined ratio mean?
The measured sides are equal, so combined contrast is zero stops and there is no measurable added key contribution in this simplified model.
Why might the photo look more or less contrasty than the ratio suggests?
Subject tone and reflectance, specular highlights, light angle and softness, flare, camera response, exposure, colour, processing, and display conditions all affect the recorded result.
Are my readings uploaded?
No. The calculator runs locally and does not send your input values to a server.