Green screens, blue screens, and chromakeys are used in many types of productions. They are most often used for background replacements on films, music videos, news casts, and comedy skits. The effect can be small, like adding depth down a hallway in an already extravagant set or can be used for a larger effect to later place a character in a completely different environment. The main goal when lighting any green screen is to light it even and simply.
LIGHTING THE GREEN SCREEN:
- Use soft sources. This will help spread the light more evenly across the screen. Since the direction of the light should be invisible the screen can be lit from above, below, the sides, or where-ever is appropriate depending on the size of the green screen and the space it is in.
- It can help to make your lighting symmetrical. Place lights equidistant from the screen and spaced out evenly between each other to keep the exposure even. It is easier to place a few lights accurately than to keep adding excess lights that will risk unwanted shadows and more unevenness.
- Check that your exposure is even. It is often said that lighting the green screen one F-stop above or below the camera's set exposure is best, but for the work I have been doing this has depended on the circumstances. For one show in particular, I underexpose the green screen a little because there is a reflective issue with a set piece that looks better if the screen is not as bright.
LIGHTING THE SUBJECT:
- Know your background. If shooting a green screen for a narrative or short film, know what is going to be composited onto the green screen before shooting. In order to seamlessly blend a subject into a green screen environment, it is crucial to have the lighting environments match. For exteriors, always consider the sun direction & intensity, color of the surrounding objects that creating bounce, and objects that move and may interact with the lighting on your subject. For interiors consider the quality and direction of any practicals or light sources in the background, as well as color temperature. For a news show with a green screen background, standard flat studio lighting works well and these considerations do not apply since the newscaster is supposed to look like they are in a studio the background environment does not effect how they look. The most important factor is that if it is supposed to look like they are sitting in front of a window overlooking a city then it is good to have an appropriate backlight.
- Watch for reflections. Make sure your actors are a safe distance away from the green screen and they are not reflecting any green light bouncing from the screen behind them. When shooting objects with reflectivity or glass, especially when using a green screen for car scenes, be careful not to have any reflections interfering with your actors.
- Framing: Be careful of the edges of the subject when shooting a subject that will later be isolated. A subject can be moved around in post once isolated from a green screen if necessary, but if the subject is accidentally cropped that could be an issue. When isolating a subject make sure the green screen covers their entire perimeter that is being used.
- Focus: Sometimes people can have problems in post if your subject goes out of focus into the green screen. Especially if a sharp image is being keyed into the green screen, you don't want a weird focus issue or a blurry edge. Keep your aperture small. When shooting a background plate and a subject separately keep in mind your iris and focus settings to help keep them consistent so the elements can blend more naturally later on.
- Movement: Don't move the camera unless you are going to place tracking points on the screen and also have a matching plate to move with it in the background.
It is important to collaborate with your post-team before going on your shoot. In the instance below we used a blue screen instead of a green screen for a night exterior in the wilderness. The idea behind not using a green screen came from the fact that most of the plants in the environment would be also green. However, due to the rapid pace of production and lack of planning, using a blue screen became a much bigger issue than the green screen would have been. Here's why...
The set had already been lit with moonlight (blue) lighting and one of our actors was wearing jeans that had some blue pigment in them. This made it hard to separate the actors from the foreground. Ultimately it would have been better to use a green screen regardless of the foliage so that the actors could be keyed properly or once locked into the blue screen, the scene in particular should have been shot with tungsten light and corrected later so as not to add blue to the actors.