Low-key lighting is any scenario in which the preponderance of the tones in a photograph is dark, set off by only a few areas of highlights. In practice, this might be a black cat in a black hat in a black room (a favorite scenario of mine, at least in theory) with just the whites of the cat’s eyes, for instance, and an edge light on the cat and the hat creating highlight separation from all those shadows.
In a low-key portrait scenario, like the one shown here, the key light might be placed far to the left side of the camera and beyond the subject plane in order to create a dramatic split lighting effect. Moved farther, almost coming from behind, the light turns into an even more dramatic edge light. In each case, significant portions of the frame—the background, as well as areas of the subject that are camera-facing—might be rendered completely in shadow, totally black. The trouble is, this can have the effect of making the subject literally blend with the background. It’s also awfully contrasty, even harsh looking. And depending on the angle of the key light, it can amplify textures in the skin, for instance, that might benefit from just a hint of fill. It could be that this harshness is the look you’re after, but more often than not a little bit of fill will help tame the extreme contrast and add just a hint of detail to the shadow side of the subject. You don’t want the fill to be very strong—likely at least two stops under the key light—because you don’t want to eliminate the drama of the key light and the low-key setup. But with just the right amount of fill—just a hint—it can turn pure black shadows into black with detail, and suddenly a vast area of simple blackness is now one of deep, dark nuance and interest. I find this look tends to be much more gratifying.
The challenge, of course, is that if you overdo the fill light you can completely destroy the impact of the low-key scenario in the first place and maybe even run the risk of making it no longer low key. In order to check this, I would suggest using a handheld meter to measure the fill light and the key light, and ensure that the difference between key with fill and the fill alone is at least two stops. If it’s too much more than that, you might not be able to see any impact from the fill, which is also no bueno. You can always use your eyes when you’re in a pinch, and remember that if the fill light looks a little too strong on the back of the camera, it’ll look way too strong in real life. Subtle fill light is the rule, here, because you just want enough light to add a bit of details to open up the shadows and help provide separation among subject, background and any other scene elements that are hidden in the darkness.
Whether you use a white card, a silver reflector or an additional light source (like a continuous light source or a strobe), you’ll want to position the fill at or near the camera on the shadow side, and move it closer or farther from the subject as necessary. A flash fill may actually work better if positioned on the key side, but very close to the camera, so that it doesn’t create a second competing set of shadows. A reflector may actually have more impact moved farther to the shadow side, away from the camera and closer to the subject. In any case, if you add too much fill light, you’ll kill the low key. And, frankly, I’d say it’s probably better not to fill it at all than to overdo it, at least in this type of low-key portrait scenario. So make sure to keep the fill light subtle and look for the subtle touch it adds to your low-key images.