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Lighting Ratio Formula

written by: •edited by: Rhonda Callow•updated: 12/11/2009

Lighting ratios can measure the ratios between the light sources on a set with an incident light meter. For instance, a ratio of 8:1 is equal to a difference of 3 f-stops.

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    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))