Calculating Studio Lighting Ratios
by Jeffrey Sward
 
typical portait lighting setup Jeffrey Sward

Lighting ratios are often used in studio photography, particularly for portraits. The assumption of lighting ratios is that there will be two light sources: the main and the fill. The purpose of the main light is creation of sculpting shadows on the subject. Generally main lights are placed at an angle other than the axis between the camera and the subject. The purpose of the fill light is to fill in shadows on the subject. The fill light is generally placed on the axis between the camera and the subject. Fill lights are often either directly above, below or around (ring light) the camera.

There are two different methods for calculating studio lighting ratios. The difficulty arises from whether to measure the ratio by the differences in the light sources or the differences in the light and dark sides of the subject (face). Because it is assumed that the main light will be casting shadows on the subject, the dark side of the subject is assumed to be getting zero stops of light. For example:

  • When both lights emit the same amount of light, the light source method makes the light-source ratio 1:1.
  • When both lights emit the same amount of light, the dark side of the subject is getting zero stops from the main light. The bright side of the subject is getting 1+1 = 2 units of light, one unit from main and one unit from fill. The dark side of the subject is getting 0+1 = 1 units of light, zero units from main and one unit from fill. This makes the light-on-the-subject ratio 2:1.

To make matters even more confusing, lighting ratios are expressed as the linear difference in light rather than in stops. Stops are logarithmic with the power of two rather than linear.

Method A: Ratio of the Light Sources

Lighting ratio = Main light : Fill light

Method B: Ratio of the Light and Dark Sides of Subject

Lighting ratio = (Main light + Fill light) : Fill light

 
Lighting Ratio Chart Both Methods
 
Main Light Fill Light Main Linear Light Units Fill Linear Light Units Main + Fill Linear Light Units Method A Light Source Ratio Method B Subject Ratio Stop Difference of the Light Source (a) Stop Difference in Light on the Subject (b)
f4 off 1 0 1 1:0 1:0 infinite infinite
off f4 0 1 1 0:1 1:1 infinite zero stops
f4 f4 1 1 2 1:1 2:1 zero stops 1 stop
f4.8 f4 1.4 1 2.4 1.4:1 2.4:1 1/2 stop 1 1/2 stops
f5.6 f4 2 1 3 2:1 3:1 1 stop 2 stops
f6.7 f4 3 1 4 3:1 4:1 1 1/2 stops 2 1/2 stops
f8 f4 4 1 5 4:1 5:1 2 stops 3 stops
f9.5 f4 6 1 7 6:1 7:1 2 1/2 stops 3 1/2 stops
f11 f4 8 1 9 8:1 9:1 3 stops 4 stops
f13 f4 11 1 12 11:1 12:1 3 1/2 stops 4 1/2 stops
f16 f4 16 1 17 16:1 17:1 4 stops 5 stops
f19 f4 23 1 24 23:1 24:1 4 1/2 stops 5 1/2 stops
 
Anomalies
 

The existence of these two methods creates many anomalies, such as:

  • Flat lighting is 0:1 using method A light source ratio and 1:1 using method B subject ratio
  • Both lights having the same light output is is 1:1 using method A light source ratio and 2:1 using method B subject ratio
  • There is no 0:1 using method B subject ratio
 
Confusion
 

The existence of these two different methods causes a great deal of confusion. Some examples:

  • Statements are made such as "when the main light and fill light have identical strengths, then the light ratio is 2:1." This statement is using both methods at random. The method A light source method is being used for the difference while the method B subject method is being used for the ratio. The following two statements are more consistent: "when the main light and fill light have identical strengths, then the light ratio of the sources is 1:1" and "when the main light and fill light have identical strengths, then the light ratio on the subject is 2:1."
  • What is the correct answer to the question: "what is the differences in stops for a 3:1 lighting ratio?" Since method A and method B are interchanged at random, the answer could be any of 1 stop, 1 1/2 stops, 2 stops, or 2 1/2 stops.
 

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