This is another installment of a long series of articles shot and composed with the help of professional photographer Karl Leopold at ImagesForever.net in Melbourne Beach, Florida. Karl is one of the top photographers in the area and president of the Atlantic Professional Photographers Association and graciously opened his studio up to us for this series.

Light ratios is one of those subjects that makes people’s eyes glaze over because it’s technical and there’s math involved. That’s why I started with the more fun Three Point Lighting instead. But to really understand studio lighting, we have get into light ratios.

In order to give a portrait depth and character, we control amount of light and the shadows. To keep a face from looking flat, we change the ratio between the amount of light coming from the main light, the key, and the secondary light, called the fill.

When applying light ratios to the subject of three point lighting, most often it will be applied to the ratio between the key light and fill, but not always as we’ll find out in later articles. Each f-stop difference equates to roughly half as much light reaching the subject. So the key and fill being perfectly matched would be a 1:1 lighting ratio.

Knock one stop off the fill either by cutting the flash power or moving it farther from the subject and that gives you a 2:1 ratio. Take 2 stops off the fill and you’ll have a 4:1 ratio.

As discussed above, you don’t usually want the key and fill perfectly balanced on a 1:1 ratio. The most common ratios used in portrait photography are a 3:1 and 4:1 ratio. So if someone told you to set up a three point lighting set with a full power 3:1 key/fill ratio at f/11 and -2 stops on the hair light, you’d know what to do. You’d set your key power to give you a metered f/11 at the subject and the fill -1.5 stops, which would be between f/5.6 and f/8, technically f/6.7 on the half-stop scale. The hair light would be two full stops less at f/5.6.

Because you need a 3:1 ratio at full power, you’d know that you’ll have to move the fill farther from the subject and use the light meter to gauge the distance. Since he’s been shooting portraits in the same location for years, Karl has marks on the floor where the fill goes for a 3:1 ratio and a string from the key to the subject for the proper distance for f/11. Once you get in the ballpark, most photographers are going to eyeball the subtle changes anyway.

The old full stop calculations are starting to give way to modern cameras that are graduated in ⅓ stop increments (1/3 EV), which yields a slightly different f-stop series, but since we’re fixing the camera at f/11, it doesn’t matter.

Let’s go back to our three light setup graphic. Notice the fill is much farther from the subject than the key? That spot corresponds to a 3:1 metered ratio with two identical floor flash units set to full power. Why would you want a full power 3:1 ratio instead of just knocking back the fill power? Maybe you want bracket the photos by raising the ratio to 5:1 and 7:1 and the easiest way to do that is just to cut the fill power instead of moving it and shooting a meter reading.

You can see in the photo the differences are subtle but noticeable. We went with a white background and darker clothing so you can focus on the lighting changes to the face. As you can see, by the time we get to 7:1, it’s looking a little dark on fill side. Not a flattering look for women.