What Are Lighting Ratios?

Today’s Post by Joe Farace

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Question: In one of your lighting reviews in Shutterbug you indicate that some the photographs have a 3:1 lighting ratio and have an exposure of 1/125 sec at f/11.  I’m familiar with lighting ratios but am weak in some aspects to the setup and execution of them. Would I be correct in believing the main light is set to f/11 and the background light is set to f/4 which gives the 3:1 ratio or did I miss something?  It seems as if the more learn the more I realize there is much I have to learn.

Answer: Lighting ratio refers to a comparison of the key or main light to the fill light. The higher the lighting ratio is, the higher the contrast of the image will be and conversely; the lower the ratio, the lower the contrast. A ratio can be determined because each increase in aperture is equal to double the amount of light or two to the power of the difference in f stops.

For example: A difference in two stops between the key and fill lights is 2 squared or a 4:1 ratio. A difference in three stops between the light is 2 cubed, or an 8:1 lighting ratio. If there is no difference at all between key and fill the lighting ratio is equal to two to the power of zero, which produces a 1:1 ratio.

For the 3:1 lighting ratio used in my shots for the review that he referred to, there is a one-stop difference. A lighting ratio of 3:1 is considered normal for color photography but photographers can be flexible in applying this rule and to tell the truth I seldom worry about hitting a specific numbered ratio.

For this shoot with Pam Simpson getting all Great Gatsby I mounted a 36-inch Westcott silver umbrella on a Elinchrom D-Lite RX monolight as fill. It was placed in the far (camera) left corner of my in-home studio; the main light used a 26-inch square softbox at camera right. Background was a Retro Blue 60×72-inch Accent collapsible backdrop from Savage. Ms. Simpson was photographed with a Canon EOS 60D and EF-S15-85mm f/3.5-5.6 IS USM (at 65mm) with an exposure of 1/125 sec at f/11 and ISO 200.

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