Lighting Ratio
Written by Abin Alex and Anu Joseph.
Abstract
A practical guide to lighting ratio: what key to fill means, how stops set the ratio, how to meter it, and which ratio suits which subject, with two diagrams and two tables.
Keywords: lighting ratio, key to fill ratio, 2:1 lighting ratio, 4:1 lighting ratio, fill light, light meter, stops, portrait contrast
What is a lighting ratio?
Short answer. The comparison of key light to fill light. A higher ratio means more contrast.
- A lighting ratio is a comparison of the key light to the fill light, written K:F (Marr, 2004, p. 35). A higher ratio means more contrast.
- The key light is the brightest light and gives the shadows. The fill light fills those shadows (Marr, 2004, p. 35). It is almost always less bright, on the opposite side from the key, and usually not as high.
Light a face with a key alone. Add a fill and watch the shadow become lighter.
On a set the same idea is called contrast ratio. A dark, moody scene uses a high ratio. A bright, soft scene uses a low one.
Cham tipThink of the ratio as how dark you want the shadow.
Cham taskWrite the key and fill roles in your own words and name one subject for a high ratio.
Check yourself: What does a higher lighting ratio give?
More contrast.
How do stops make a ratio?
Short answer. Each stop doubles the light, so a difference of n stops gives a ratio of 2 to the power n.
- Each stop doubles the light (Grey, 2004, p. 23). A move from f/4 to f/5.6 needs double the light. Two stops is four times the light, and three stops is eight times.
- The ratio is therefore 2 to the power of the difference in stops. A one stop difference is 2:1, two stops is 4:1 and three stops is 8:1 (Grey, 2004, pp. 24-25). Figure 1 shows the fill as a share of the key.
- The ratio depends on lighting power, not exposure value. If two identical lights are at the same distance with the same modifiers, a key at full power and a fill at half power gives 2:1.
Set a fill to half power, then a quarter, then an eighth. Meter each time.
A stop is a stop on a camera and on a light. Cutting a light by one stop halves it.
Cham tipCount stops, not watts. Stops are what the camera sees.
Cham taskWork out the ratio for a difference of one, two and three stops.
Check yourself: What ratio is a two stop difference?
4:1.
How do you measure a ratio?
Short answer. Meter the key alone, meter the fill alone, and count the stops between the two readings.
- Meter the face with the key light alone. Then meter it with the fill light alone (Marr, 2004, pp. 37-38). The difference in stops is the ratio (Grey, 2004, pp. 24-25).
- For 1:1 both read the same, for example f/8 and f/8, and no shadow is visible. For 2:1, adjust the fill or reflector distance until it reads f/5.6 against an f/8 key. For 4:1, the fill reads f/4. For 8:1, the fill reads f/2.8. Figure 2 shows the 2:1 case.
Switch off the fill, meter the key. Switch off the key, meter the fill.
A spot meter on the lit and shaded side of a face does the same job on set.
Cham tipMeter from the subject toward each light, one light at a time.
Cham taskSet a 2:1 ratio and record the key and fill readings.
Check yourself: What does the fill read for a 4:1 ratio against an f/8 key?
f/4.
Which ratio should you use?
Short answer. 2:1 and 4:1 suit most portraits. Go lower for babies and higher for drama.
- A higher ratio is more dramatic and gives darker shadows. The best ratios for most portraiture are 2:1 and 4:1, because they model the face without unattractive deep shadows (Marr, 2004, p. 39).
- Babies and small children suit 1:1 or 2:1. Women suit 2:1 or 3:1. Men and business portraits suit 3:1, 4:1 or 6:1, which is two and a half stops. Artists, bands and drama suit higher ratios. 3:1 is good for black and white. Table 1 gives the ratios and Table 2 the subjects.
- Squint at the subject before you shoot. Squinting shows roughly the narrower range the sensor records, so you see which shadows will go too dark and need fill (Marr, 2004, p. 39). Too much fill, though, makes the picture flat (Marr, 2004, p. 41).
| Ratio | Stops | Look | Use |
|---|---|---|---|
| 1:1 | No difference | Flat, even | Flat lighting |
| 2:1 | 1 stop | Gentle | General colour |
| 3:1 | 1 1/2 stops | Modelled | General black and white |
| 4:1 | 2 stops | Dramatic | Low key |
| 8:1 | 3 stops | Very dramatic | Very dramatic low key |
Table 1. Lighting ratios compared.
| Subject | Ratio |
|---|---|
| Babies, small children | 1:1 or 2:1 |
| Women | 2:1 or 3:1 |
| Men, business portraits | 3:1, 4:1 or 6:1 |
| Artists, bands, drama | Higher |
| Black and white | 3:1 |
Table 2. Ratios by subject.
Photograph one subject at 2:1, 4:1 and 8:1. Choose the one that suits the mood.
A thriller uses a high ratio. A comedy or a family drama uses a low one.
Cham tipIf the shadow hides the eye, lower the ratio.
Cham taskPick a ratio for a baby, a business owner and a band and explain each.
Check yourself: Which two ratios suit most portraits?
2:1 and 4:1.
How do you set the ratio with your lights?
Short answer. Place the lights 45 degrees from the camera and move the fill closer or farther until the reading matches.
- Position the lights about 45 degrees from the camera. The greater the angle, the greater the contrast.
- Change the ratio by moving the fill closer or farther, by changing its power, or by moving a reflector. Meter again after every change.
Start with the fill beside the camera. Move it back until the reading is one stop under the key.
Moving a light farther away, or adding a flag or net, lowers it by stops on a set.
Cham tipChange one thing at a time and meter after each change.
Cham taskHit 4:1 by moving the fill only, and record the distance.
Check yourself: What happens to contrast as the light angle grows?
It rises.
Questions beginners ask
What is a lighting ratio?
A comparison of the key light to the fill light, written K:F. A higher ratio means more contrast.
What is a 2:1 lighting ratio?
The key is twice as bright as the fill, a difference of one stop. For example the key reads f/8 and the fill reads f/5.6.
Which lighting ratio is best for portraits?
2:1 and 4:1 suit most portraits. Babies suit 1:1 or 2:1 and dramatic portraits suit higher ratios.
How do you measure a lighting ratio?
Meter the face with the key alone, then with the fill alone, and count the stops between the readings.
References
- Grey, C. (2004) Master lighting guide for portrait photographers. Buffalo, NY: Amherst Media.
- Marr, D. (2004) Beginner's guide to photographic lighting: techniques for success in the studio or on location. Buffalo, NY: Amherst Media.
Further reading
- Begleiter, S.H. (2008) 50 lighting setups for portrait photographers: easy-to-follow lighting designs and diagrams. Buffalo, NY: Amherst Media.
- Child, J. and Galer, M. (2008) Photographic lighting: essential skills. 4th edn. Oxford: Focal Press.
- Siskin, J. (2011) Understanding and controlling strobe lighting: a guide for digital photographers. Buffalo, NY: Amherst Media.
How to cite this guide. Alex, A., & Joseph, A. (2026). Lighting ratio: A guide for photographers and cinematographers. Creative Hut Academy. https://creativehut.org/library/lighting-ratio/
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