Here are a few tips for using the backlight effectively in a four point lighting set up.
Function of the Backlight
When filming an individual, whether in an interview setting or just a narrative still frame, you will be using a multiple light setup. Conventionally, the four point lighting setup is used. With the four point lighting setup you will place a key light on their face that is sharp, a fill light on the other side of their face that is a little softer, the background light to light the background behind them, and the backlight that lights up the back of the person and cuts them out from the background. This backlight, also called an edge light, is one of the ways that you really defy the limits of this two dimensional medium. The backlight is often left behind, but is actually pretty essential for really creating a professional digital video image. Here are some tips for setting up a backlight that will really make your four point lighting setup work.
Placement According to Other Lights
One of the best tips for the backlight is to actually make sure that it fits well into the four point lighting setup. What this means is that you will begin with the key light, match up the fill light, and then place the backlight. What you will do is keep the backlight directly to the diagonal of the key light and on the same side of the camera as the fill. This way you will get a good balance between the backlight and the key light and it will keep the backlight into a proper perspective for the camera.
An important backlight tip is for matching the intensity of the backlight to the perspective for the project itself. This means that for a flatter scene that does not require an intense amount of drama, like a dialogue oriented comedic scene, then you may want to have the backlight be a little less intense. If it is a very serious interview in a documentary, or somewhat surreal, then you may want to make the backlight very intense. When altering the intensity you can follow a basic backlight tip for using the rotational abilities of the light itself. Instead of repositioning the light you can often just rotate it more or less onto someone's head. If you turn it more toward their head it will increase the intensity, and if you rotate it away then you will lower the overall intensity. Beyond this you may want to use your barn doors and begin applying some scrims to lower the stops.
Position and Angle
The position and angle of the backlight is going to be important for creating a good set up and edge illumination. This is going to be different for every single different set up. For example, you are going to have to be creative when getting a backlight onto a character in a wide shot where you can see their whole body. For this you are going to have to try several different angles and positions, both to get the desired effect and to get it out of the digital video frame.