Lighting Ratio Formula
How To Find Lighting Ratios
A lighting ratio is the ratio between the key light and fill light, not the brightness reflecting off the subject ( this would be a brightness ratio/ contrast ratio which is determined with a reflected light meter instead). Like brightness ratios, lighting ratios do play a very important role with establishing style and mood, but in this article we’ll look at the mathematical approach in determining lighting ratios.s
To determine a lighting ratio between lights on a set, first, an incident light meter reading is taken of the key light. Then, a meter reading is taken of the fill light. The reading will determine the appropriate aperture setting on your camera. Now that you’ve got a reading of both light sources, you are able to determine the lighting ratio.
For example, let’s say your incident light meter gives you a properly exposed key light reading of f11 and a fill light reading of f5.6. What would be the lighting ratio? (assuming you already know the basics about f-stops and reading light meters)
You first may determine the f-stop difference between both light sources. Since f11 and f5.6 are a difference of 2 f-stops, then the lighting ratio is 4:1.
How do you determine this? It’s a simple equation!
*Raise the number 2 to the power of the f-stop difference (ex. 2 ^ 2 (f-stop difference) = 4 = 4:1 ratio ; 2 ^ 3 (f-stop difference) = 8 = 8:1 ratio, etc.).
In the above example you raise 2 by 2 (the f-stop difference) to get 4, which is 4:1, the ratio.
common ratios (lighting and contrast):
1:1 (no difference) (flat)
2:1 (1 stop) (common Television standard/ high-key lighting)
4:1 (2 stops) (common in still photography portraits)
8:1 (3 stops) (low-key lighting begins)
16:1 (4 stops)
32:1 (5 stops)
64:1 (6 stops)
128:1 (7 stops) (film contrast ratios)
* stops in between are stops and a half (for example, 3:1 (1.5 stop) and 16:1 ( 4.5 stops))