When photographing a human face, your primary goal is to make your subject look good. Your job, in most cases, is to simply accentuate their positives and hide their flaws. Most portrait photographers know that a large, soft light source (like open shade, window light or a softbox) is a great way to create an appealing, flattering portrait light source. But that’s only part of the challenge. Where you position that light changes quite a bit in the end result.
For instance, a frontal light source—if it’s soft enough—can be an appealing way to minimize harsh shadows and textures, and generally creates a pleasant portrait light. If your subject is large, however, or has a particularly large face, such frontal illumination is going to accentuate that. Moving the light to 45 degrees—or even to the 90-degree position to create “split lighting”—is a great way to create a slimming effect by making a round face appear more narrow. Allowing that subject to emerge from a dark background is another technique for subtle thinning. A backlight, in that case, can serve to separate the subject from the background by creating a rimlight effect. This is a good way to keep the subject from blending into the background, but keep in mind that blend is what helps to create the slimming effect.