If a white card placed opposite the key light bounces light into the shadows, a black card does the opposite. It’s a simple way of effectively pulling light out of a scene and adding shadows where there are none.
When is negative fill most useful? It’s especially effective in scenes that are high key—that is, scenes filled with light and bright tones. If I’m photographing someone in a light-colored sweater against a bright background, I’m bound to use negative fill opposite the key light in order to add a shadow that defines the edges. In practice, I find that any time I’m photographing a subject against a white background in studio, negative fill becomes especially important to create separation—whether it’s a product shot, a portrait or really anything at all.
So what works for creating negative fill? It can be a black flag like you might see on a movie set—a metal frame wrapped in matte black fabric and attached to a c-stand—or just a piece of black foamcore. Either way, the effect is similar: removing light from a place with too much.
You can even continue this technique outside of the studio with natural light. Working outdoors, dark-painted structures and tree trunks can act as negative fill. Of course in these situations you have to move the subject close to the structure rather than the other way around. Try it the next time you’re making a portrait in the park. Stand a subject close to a tree trunk, and move their face closer and closer to flag light and create that negative fill. Whether you do it in the studio or out, with a flag or a tree, the effect is the same—to provide a well-defined shadow edge and help separate a light subject from a light background.