The classical definition of lighting ratio is that it’s a comparison—a quotient for all the mathematicians out there—of the main aka key light to the fill light. In simplest terms, the higher the lighting ratio is, the higher the contrast of the image; the lower the ratio, the lower the contrast. In classical photographic portraits, a lighting ratio of 3:1 is considered normal for color photography.
Using a kind of standard lighting ratio is helpful when getting started because it provides a baseline guaranteeing your images will have some sense of lighting rather than the bland “lets through every light at it” approach. But there are pros and cons to using any kind of formulas in photography, especially when it comes to lighting lighting.
There are at least two approaches to setting up lighting ratios: One is to let the ratios control the shot and the other, which happens to be my favorite, is to set the lights, meter the exposure (or not) and worry about the lighting ratio later. As always tou can use whatever approach fits your working style but if you’re at all interested, here’s how I do it:
I start by placing the lights based on what the model is wearing, what kind of poses I’ll be shooting, and how much shooting space is available. In general I prefer to work with the softest lights possible. My method also depends on the lights having proportional modeling lights so changes in lighting are reflected in their output too.
That’s why this method works for continuous lighting sources as well. Then I used the light’s variable output settings to vary the light to produce a ratio that looks good to my eyes, as I did with this portrait of aspiring model Laura Bachmayer. You can see the simple two light set-up, above right.
Then I use a flash meter (or not) to determine an exposure and then make a test shot and evaluate the image’s histogram. If I need more light I’ll often use the camera’s ISO setting increasing it slightly so I can keep the depth-of-field exactly as it was in the test shot. With most of today’s cameras there is little difference in digital noise whether the ISO is set at 100 or 400.
So, for me, the concept if lighting ratios is more important than their application. But as always with this blog—it’s you decision not mine.
There’s more on working in the studio, no matter where it might be. You can learn more in my book, “Studio Lighting Anywhere.” Please pick up a copy for yourself or a friend. It’s is available from your favorite book or camera stores as well as including Amazon.com.