Lighting ratios were especially important in the days of film, because the limited contrast range meant you had to be especially precise when balancing shadows and highlights. With RAW digital capture, it seems like we can almost always pull detail out of the darkest shadows, and rescue all but the brightest highlights from becoming blown out pure white. But in reality, RAW files still have their limits. And, to make the ideal exposure in camera in order to maximize detail in shadow and highlights, as well as the ideal overall "look" of an image, mastering lighting ratios is as important as ever.
Most commonly, specific lighting ratios are useful in portrait lighting because they express a contrast value between the highlight side of the face (the cheek closest to the main light) and the shadowed side (the cheek opposite the key). And, because those ratios say a lot about the lighting style and overall look and feel of a portrait. The higher contrast of a strong ratio packs more drama than a lower ratio's flat illumination.
When measuring the amount of light falling on a subject, a doubling (or halving) of a light's intensity is equal to one full stop. That means a 2:1 lighting ratio represents a subject with a highlight side and a shadow side that are one stop different. By extension, 1:1 is perfectly flat—no difference in brightness between highlight and shadow. So, 2:1 is one stop, 4:1 is two stops, 8:1 is three stops and so on. When working with film, photographers had to keep their ratios fairly tame, often a 4:1 or 5:1 ratio at most, to ensure holding detail in the shadows. With wide digital latitude, stronger ratios are possible, and subtle differences are often more evident.
Want to create flat lighting scenario to, for instance, make a thin face appear broader? Add fill light to the shadow side of the subject in order to bring the ratio to 1.5:1 or even 1.1. Or, do you want to make a face appear thinner? A stronger ratio (perhaps 4:1 or 8:1) and split-lighting approach may be just the ticket. Care to make that subject appear to emerge from the shadows for a dark and mysterious effect? The four-stop difference of a bold 16:1 ratio might be the perfect place to start.
The handheld incident light meter is the best way to measure a lighting ratio, but these days those things are few and far between. To measure lighting ratios with your camera, position a gray card first on the main light-illuminated area of the subject—the brighter cheek in a portrait, for instance. Fill the frame with the gray card (taking care not to cause a shadow) and record the appropriate reading via your camera's TTL meter. Let's say it's 1/125th at f/8. Without repositioning the subject or lights, move the gray card to the shadowed cheek and repeat the process.
A 3:1 ratio is a very classic portrait ratio, and would be reflected here with a shadow side reading of 1/125th at f/4.5—that's one-and-a-half stops darker than the main light. If your aim is to achieve that 3:1 ratio, but the light just isn't quite bright enough (say, the meter reads only f/4) add fill (via reflector or a stronger fill light) to bring it up a third of a stop to f/4.5, thus creating the 3:1 ratio that is so classically pleasing.
Even if you don't want to go to the trouble of using a gray card and measuring lighting ratios, understanding what ratios represent is a great way to help you think about contrast and mood in your photos, and take control of both. Here's a rundown of lighting ratios revised for the era of the RAW digital image file.
1:1 - Flat lighting. Very even, from shadow to highlight. Can occur with a single light placed frontally, or with a sidelight and a too-strong fill. A 1:1 ratio is not typically desirable in portraits, perhaps because it accentuates round faces and simply isn't as "interesting" as a stronger ratio. The best exception to this rule is with bright, high-key portraits in which a strong ratio would be unpleasing.
2:1 - One stop. Although a 2:1 ratio produces a shadow that is a full stop darker than the highlight side, in practice this is still a fairly minimal portrait ratio. The difference is there, but it's minimal. And, it's not going to create much of an impact beyond flat lighting except to add a touch of interest to let the lighting pattern start to emerge.
3:1 – 1 & 2/3 stops. The 3:1 ratio is considered a classic portrait ratio, very pleasing to the eye. Shadows are easily evident, but not so dark as to become particularly dramatic. A subtle adjustment of a reflector is usually enough to take a flat 2:1 ratio—or even a contrastier 4:1 ratio—to the 3:1 standby.
4:1 - Two stops. A 4:1 ratio might be useful when a touch more drama is wanted compared to the flatter ratios above. I find that when I want to give someone a more unique, more artful look, a slightly stronger ratio with bolder shadows is often helpful. In the film era, a 4:1 ratio would border on too strong, but with RAW shadow detail at your disposal, a 4:1 ratio is not dramatically different than a 3:1 ratio.
8:1 - Three stops. Although it sounds dramatic, an 8:1 ratio is often quite attractive in digital portraits, especially when used in conjunction with a Rembrandt or Loop lighting pattern. The increased drama of this three-stop ratio is a great way to take a traditional portrait from bland to intriguing.
16:1 - Four stops. Now we're approaching special effect territory. The big four-stop difference from shadow to highlight in a 16:1 lighting ratio is really for very specialized purposes—making someone look mysterious or sinister, for instance. Of course, that's if you're using mostly shadow and a bit of highlight to achieve the ratio. The reverse—a mostly bright image with only occasional areas of deep, dark shadow—is bold and contrasty in an entirely different way. However you approach it, the digital SLR's ability to hold highlight and shadow detail makes this four-stop range possible with precise RAW processing.
32:1 - Five stops. If you want to ensure that the shadow side of a subject blends in with a pure black background, a ratio of 32:1 (or five full stops) is the best way to get there. It's got to be employed precisely and only on special occasions, but it does have its place.
Remember, you don't have to be a studio portrait shooter to harness the power of lighting ratios, and you don't have to stick to these fixed full-stop ratios. Anyone who lights anything, from architectural interiors to landscapes, can benefit by using ratios to understand how strong a shape will appear in the picture, and whether or not additional fill light is needed to decrease that ratio and boost shadow detail.