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05.04 How to Use a Green Screen

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A green screen is a useful tool for shooting footage designed for chroma keying .

Basic Green Screen Setup

Green Screen Preparation

The green screen should be a solid color green. Some green screens are permanently installed on solid walls and have no wrinkles except for dirt on the paint.

If using a fabric green screen, make sure it does not have any wrinkles or folds. If you can, use a wrinkle-free green screen material that is specifically designed for chroma keying. Fleece material works well to avoid wrinkles and to diffuse the green light. Make sure the fabric is pulled taught and is smooth.

Use a green screen that is larger than you need. This will give you more room to work with a moving subject and allow for more separation between the screen and the subject.

Lighting the Green Screen

The green screen should be evenly lit so that there are no shadows or hot spots. This will make it easier for the chroma key software to distinguish between the green screen and the subject. Use a diffuser on your lights for the green screen. This will help to soften the light and create a more even lighting pattern without shadows or hotspots.

The subject should be lit separately from the green screen. This will help to prevent any green spill onto the subject, which can make it difficult to key out the background. Use a light on the subject or a reflective board to reflect light back onto the subject. This will help to fill in any shadows and create a more dimensional look for the subject.

Think about what the final scene will be before lighting the subject. The lighting of the subject should match the lighting of the final scene. If they will be in the middle of the street with a car’s headlight approaching from camera left, it will be a problem if the subject is lit from the right during the green screen shoot. If it makes sense for the planned final video composite scene, the addition of a rim light, shining from the rear of the subject, can help separate them from the background.

Subject Placement and Preparation

The subject should be placed at least 3 feet away from the green screen. This will help to prevent any green spill onto the subject. The colors green and blue are often used for chroma keying, so it is best to avoid wearing these colors when shooting against a green screen.

To help avoid green spill, black fabric can be placed under and around the subject to cover the green screen that is not seen on camera. This can help stop green reflections from spilling onto the subject.

Camera Setup

Depending on the aperture of the camera and the needed depth of field, it can be a good idea to have the green screen slightly out of focus to blur together any imperfections in the green screen. A lower aperture will have a shallow depth of field and will keep the subject in focus with a blurry green screen. Make sure the depth of field is not too shallow making the subject out of focus.

Using a high shutter speed can help avoid motion blur. Motion blur is difficult to key out in post production. Shots without motion blur can look unnatural, so after the keying is done, ofter motion blur is “added” with visual effects software.

Use a lower ISO to avoid grain and noise in the shot. This is a balance between the needs for shutter speed and aperture.

Rotate the camera as need to fill the frame as much as possible with the subject to get the most resolution. For example, if a subject will be standing in a full shot, don’t shoot in landscape mode. Instead rotate the camera to portrait mode and fill the frame with the subject.

Shooting the Footage

It is best to have a plan. Even if it is small sketches of the camera framing, planning helps avoid mistakes and get the shots that you need.

If shots involve camera motion that needs to be motion tracked, then make sure you place tracking markers on the green screen. Try to place the tracking markers where they will not be overlapped by the subject since that will be harder to remove in post production.

During shooting, make sure to review the footage to make sure the subject did not accidentally go past the green screen during movement and wild action. This defeats the purpose of setting up a chroma key shot.

Planning is key. Think about the final shot. If the subject will be in an epic background, then you probably don’t want the camera close to the subject with a wide angle. Instead move the camera back away from the subject and zoom in to fill t he frame. The same holds true for camera height. Make sure the camera height and perspective used in the green screen shoot matches that of the planned composite scene.

Green Screen Shutter Speed Shot Test Exercise

To see the way different shutter speeds change the amount of motion blur and difficulty of keying the footage, shoot sample footage a different shutter speeds. Then try to key out each piece of footage. Footage with more motion blue may look for natural but could be harder to key. An easily keyable scene with no motion blur may seem unnatural. This way you can balance your needs based on the shutter speed test.

Shutter Speed Tests

For 30 fps shoot at:

  • 1/30
  • 1/60
  • 1/125
  • 1/200

For 60 fps shoot at:

  • 1/60
  • 1/125
  • 1/250
  • 1/320