What is a green screen and how does it work?
Green screen—sometimes referred to as blue screen, or as it’s technically called, chroma key—is a digital post-production technique for compositing, or layering, different video streams based on color hues. In other words, a green screen lets you superimpose one photo or video stream over another, making it look like a single stream.
Using a green screen involves shooting your subject against a solid color backdrop, usually bright green. Using editing software you then digitally remove, or key out, that color based on its chroma range—a process known as chroma keying. You can then drop the isolated subject onto a new background of your choice.
This way, footage shot in a small studio, or a closet draped with green-screen, can appear to take place in a sunlit valley or busy streetscape—and, if done correctly, can look very convincing.
Why use a green screen?
Using a green screen is a time- and budget-friendly alternative to building an elaborate set or renting a studio. Your ideal shooting location might be too expensive, too busy, or inaccessible. A green screen makes shooting “anywhere” possible—so any filmmaker on earth can set a scene on the moon. And any filmmaker on the moon can set a scene in Indianapolis.
It’s also useful for delivering information visually. TV news meteorologists use green screens to report the weather forecast, pointing to parts of a map and showing weather patterns in motion. Not only does this make it easier for the viewer to grasp the information, it also personalizes the broadcast and helps connect with the audience. Plus, it’s super cool.
Consider using a green screen to replace the background of your video with a graphic, photo, or video element if you’re, say, recording presentation slides or your desktop, or making an instructional video.
Why must the background be green?
You might be wondering why this technique calls for a green background as opposed to another color. For one, green doesn’t match natural skin tone or hair color, making it easy to remove without compromising your subject and/or other elements in the foreground. Green requires less light, registers brighter on electronic displays, and works well for outdoor settings where a blue screen might match the sky. It’s also less common in costumes. If you can’t use green—if you have a green prop or one of you characters is a Martian, for example—use blue, which is considered second best.
How to DIY a green screen video effect video
Shooting with a green screen can save time and resources, but if done incorrectly it can create more work or unusable footage. Here are a few technical tips to help make your green screen experiments fruitful.
The basic equipment you need for the green screen effect is:
- a green backdrop, which can be a piece of fabric
- a high-resolution camera
- a tripod
- video editing software with a chroma key tool
Before setting up your equipment, decide whether an indoor or an outdoor setting allows you to create a better lighting situation. If you aren’t working with powerful lights, consider shooting outside in sunlight or overcast daylight, but keep in mind that you will be at the mercy of the weather and sudden light changes. An indoor setting, like a studio, provides a more controlled creative environment, giving you better command of your lighting.
Keep your camera completely still with a tripod. Make sure your shot centers on the green screen, which should cover the entire background of your frame. It also helps if your green backdrop is flat and smooth. Even slight wrinkles can cause issues in post production. Iron out creases.
Use a high-resolution camera. It should shoot HD-quality video, meaning 720p or higher, and with a frame rate of 24 fps or more. Most phone cameras are capable, but may not produce the best results. Shoot in MP4 or MOV format, and check that your video editing software is compatible.
Lighting is crucial. A properly lit green screen makes keying out the green in post production easier. Light your screen thoroughly and entirely before lighting your subject. They shouldn’t be treated as one.
Expose the green screen backdrop evenly, making sure it has no dark or bright areas. To avoid shadows, try for diffused, non-directional lighting on the entire screen. Diffusion spreads light, evening it out, creating a soft, gentle effect. You can diffuse light by shining it through semi-transparent material, like a softbox or white bedsheets, or by bouncing it off a reflective surface.
If you have the resources, start with two 1000-watt lights with softboxes for the screen, and a 650-watt light and a 500-watt light with softboxes for your subject. You might also want a backlight to help highlight and define your subject’s outline.
Last but not least, consider the type of footage you’ll be using for your new background. Light your subject accordingly, paying particular attention to light direction and color temperature. Light appears cool or warm depending on the kind of bulbs you use. If your new background clip is a sunlit valley, the last thing you want is a subject lit with fluorescent dental-clinic-like light, or, worse, for the sunlight to be coming from the wrong side.
Screen and subject position
Keep as much space as possible between your subject and your green screen. Not only will this help mimic the lighting of your new background on your subject, it’ll help prevent the screen’s green tones from bouncing off the screen onto your subject. A larger backdrop can help you achieve a greater distance. The focal length of your lens can also be a factor.
Clothing and props
Remember, you’ll digitally remove—or key out—the green of your backdrop based on its color hue—or chroma range—in post production. Any other green objects in the frame, or anything the color of your screen, will also disappear. Make sure your subject or subjects are prepared and dressed to “show up”. That means no green.
In fact, the further clothing and props are from green or blue, the better. Colors with just a hint of green can appear semi-translucent. Reflective objects and surfaces can also be a problem. Try removing any shine from your subject's face or head.
Editing green screen videos
There are many video editors that can help you key out your green screen recording, some better than others. Free options include Windows Movie Maker and iMovie. Keep in mind that consumer grade software tends to have lower-grade chroma key tools. Sophisticated editing suites such as Adobe Premiere Pro or Final Cut Pro X (which has a free trial period) usually produce better results but can be difficult to navigate for beginners.
Descript doesn’t have a chroma key tool — yet. Stay tuned on that. And use Descript to edit pretty much any other kind of video.
You can also select the green screen clip and narrow down the footage you want with a rough edit in Descript, then export your remaining footage to another editor. You don’t want to spend time making adjustments on footage you won’t use.
Next, key out the background of your video with the appropriate tool, depending on the software. With a properly lit, well set-up shot, the software should key out most or all of the background. Adjust the settings to optimize your key, then make necessary adjustments to your new background. Consider adding depth of field by applying a blur filter.
The bottom line
A green screen can produce Hollywood-worthy results, and may seem like something reserved for professionals. It’s not. With the right lighting and the right setup, you can easily—and affordably—marvel your audience like Méliès.