Here are a few tips for balancing the key light and fill light on a subject when using a three point lighting set up.
Three Point Lighting
When you are filming in a studio set up, whether in an actual film or television studio or just one constructed in an apartment bedroom, you are likely going to be using a lighting set up with artificial lighting. These lights can be built in as they often are in a fully equipped film studio or brought in through portable light kits. Each lighting set up is unique depending on situation, needs, and available video equipment, but often times people will follow a general schematic of a lighting type.
One of these, which are often used for documentary interviews, is the three point lighting set up. With this you position two or three lights, a white reflecting card, as well as the camera and background to create a well lit environment. The main light that is positioned on the subject’s face is the key light, while the white card on the other side is the fill. You then need a more pronounced light on one side of the subject’s face and you position a clamped up white card just close enough to put a soft glow on the other side of their face, bringing out their skin tone. When you are going this it is important to find the perfect balance so that each plays into the other.
Key and Fill
Right off the bat you have to decide how dramatic you want to falloff, or shadow contrast to be. To get more dramatic you can bring the key close to the face and back up the fill, and to soften it up you can perform the exact opposite function. If you want to lower the intensity of the key light you can close the “barn door" shutters a little more, turn the key light out away from the subject, or put a filter or gel of some sort over the light. Remember that a gel will color the light a great deal so you may need to alter the background, make up, clothing, and white balance.
You are going to have to reposition the entire set when balancing the key and fills. The backlight is going to illuminate the back of the subject’s head and can overshadow the fill. If you want the front of their face to be exclusive, especially with the fill side going against the key side of the face, you can reduce the backlight by turning it away from the subject. You can also back up the background objects so that the background light on them stays far away from the subject.