Here is a brief guide to using negative fill in your digital video lighting.
Deleting Spare Light
Lighting for digital video and motion film is actually the culmination of how much light is in a situation and where it is placed. This means that lighting does not just refer to the lights you place and how you direct them, but also areas where there is no light. When you are setting up a lighting situation you may notice that you need to reduce light in certain areas, such as the falloff from the edge of one light or the ambient light in the room. To take care of some of this soft light you may have to use a process called negative fill.
Reversing the Fill
Negative fill, as the language seems to indicate, is the direct opposite of fill. A fill light "fills" in the gaps set by the more dramatic and focused lights, such as the key light. It is a softer light that will spill out over a larger area, spreading out the amount of surface that is illuminated and giving a more diffused blanket of light.
Negative fill, as the inverse of that, is meant to absorb light and take it off of an area. A fill light will often simply come from a reflecting surface or a white card. Negative fill will usually be a large sheet of black cloth that can be placed in a certain area. This negative fill area can be placed in the absence of a fill card so that you can have the reverse effect. For example, if you want even more dramatic shadows on the opposite side of the face from where the key is then you may want to put a black card there for negative fill.
Using Negative Fill in Digital Video Lighting
Negative fill is going to be less of a dramatic control of light than regular fill since it does not really direct light in any practical light. Using black surfaces even in the frame can act, to a degree, as negative fill. Many reflectors will have a black surface on them specifically to be used as negative fill or block light sources. Often times you may use a flag or cutter to block out the excess light, and these large black cloths can easily be used as a negative fill. The amount of negative fill that you are going to need is going to be directly related to how much light you want to absorb, the situation you are in, the light sources you are dealing with, the type of digital video camera you have, and the surface that you are going to be using to achieve negative fill.