Photographers use light to create definition in the subjects/objects we photograph.  Generally speaking, whether you are creating a portrait of people or photographing a still life, you need a solid understanding of light ratio in order to properly light your subject matter; we use shadow as a definition tool for lighter subjects/objects and highlights to define darker subjects/objects  One could say that photographers use light similarly as a make-up artist will use make-up on their subjects to define their faces.

Light Ratio

Photography is defined as painting with light. So, obviously light is light, but it is all about what we as artists do with it.  Basically we create Light Ratios by combining multiple sources to derive our final image.  What exactly is light ratio? Simply put, light ratio is contrast.  The intensity is your variable. Let’s consider that we are building a building.  First we need a foundation or a base to make our building.  The foundation in terms of lighting would be our Fill, or Base Light.  Light ratios are always built relative to the base or fill light.  The fill/base light always has a value of one.  Remember, unless you are using a reflector, that the fill light almost always comes from a source directly behind the camera vantage point. This insures the fill light strikes the surface of everything the sensor/film plane sees (fig 1).  It is important to remember that all lights that are used in images should be on one side of the imaginary 180° line we draw from the camera point thru the subject.  We only have one sun in our solar system therefore we must replicate that light pattern.  Once we cross the 180° line we create a second shadow path which becomes inconsistent with our solar system pattern.

Building Light Ratio

Since we have established our base, let’s add the main source of light.  As soon as we do this we are creating a ratio.  In figure 2 you will see that the main source of light does not directly hit the subject everywhere that is visible to the sensor/film plane.  This area, which is in blue only, has the relative light value that is created by the fill light.  Everywhere else on the subject has a combined value of both the main and fill light sources.

We are going to get slightly technical and stray from theory for a moment.  Let’s say we know the value of the fill light is f5.6 if we add another light that has the same value of f5.6 what is the proper exposure in the highlight area?  If you said f6.5 you are correct.  Each time we add the same value of light to itself we increase our exposure ½ stop.  If we apply these values to figure 2 everywhere the red and blue touch have a value of f6.5 and where only the blue touches is a value of f5.6 resulting in a 2:1 ratio of light. Complicated? Not really.  Remember the fill has a value of one, and because the main has the same value of one.  Therefore we can conclude that where ever both lights touch the subject we can add the fill’s value, one,  to the main’s value of one resulting in two. Let’s apply this theory to another set of variables.  The fill light again measures f5.6 and the main light is twice as strong and measures f8.  Because f8 is twice as strong as the fill we add the value of the fill to the main and derive a 3:1 ratio.

Exposing for Light Ratio

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The Light Ratio Chart below is constructed so you can easily calculate light values and build the ratio you desire.  Let’s say you want to create a 5:1 ratio and you want to have your fill light set at a value of f5.6.  Remembering that the fill light has a value of one follow across the top row and find f5.6 then go down that column till you intersect the 5:1 row and you will find that you need to set your main source of light at f11.5 to achieve your desired result.

Now that you can see how light ratios are built you might ask how do you practically apply them. Basic rule of thumb is to base light ration on you subject to your background.  For instance you will want to you a ratio with less contrast in a high key (all white) setting and a ratio with greater contrast with a low key (dark) background. Check back for part 2 of this series where we will review portrait light patterns. Visit us online at larmonu.larmonstudios.com to register for private mentoring at Larmon Photo or a portrait lighting class at Foto Forum.