Lighting is more about control of light rather than the spontaneous act of light creation. Lighting for digital video does often encompass different types of light kits and sources, but all of these elements work in concert with different types of barriers. To get the adequate amount of light you are looking for in the right areas you are going to have to separate that light so that it is in the right locations. Once way to do that is to utilize a flag.
A flag, which is also commonly called a cutter in digital video lighting, is used to block off the expansive spill of light. A flag is presented as a separate, monolithic object from the light source. This object usually consists of a large piece of black cloth, preferably mad of felt or some other absorptive piece. This flag is then rigged to a c-stand so that it can be positioned in front or to the side of the light source. The flag is then positioned from here to block out the light that you are looking to eliminate.
Controlling the Flag
Every light source will spill light around to different areas other than its intended target. Therefore you place one of your cutters to block the extra areas. Often times when you take a specific light, especially a high powered open face life with nothing but a thin scrim protecting the bulb, This light will spill out in all directions, not just on the subject that you want. Move the flag up to “cut” the beam of the light, only allowing a certain segment of it to spill out undirected. This will be perfect to cut the light from going just to one half of the room.
When positioning your flag, or cutter depending on production situation, you are going to have to decide how you want to position it in relation to your light. There is a tendency to place the flag to one side of the light and then either in front of the light or directly to the side of the light. Instead of these two extreme positions you want to put the flag just a little in front of the light and to the side. From here you will continue to adjust until you get the lighting barrier and falloff that you want. The last thing that you want to do with a flag is to present a shadow or limit the intended light on your subject.
When you are dealing with a natural light source, such as sunlight coming through a window, you are going to have an even more tough time ahead. In these cases the size of the flag may need to be different as it will be dictated by the expansiveness of the light opening. You are likely going to have to readjust the entire set to accommodate for the blocking features of the flag. You may have to try several different repositioning.
Using a flag on an actual light is much easier because a light is a small light surface that sends light out expansively, as opposed to a window that starts out large. The other problem you are going to see when positioning a flag with a window is that the exact direction of the light will change over the course of the day.