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Cinematography3 min read

LESSON 106

Shooting for green screen

Light the screen, then the person.

IN THIS LESSON

  1. The aim
  2. Even light on the screen
  3. Spill and matching
  4. Practice: a set-up that works

The aim

Chroma keying replaces a single colour behind the subject with another picture. Our earlier lesson on chroma key explains why green and blue are used; this one is about shooting so the key works. The most important factor is colour separation between the subject and the screen. Blue or green are chosen because they are furthest from skin tones, and green is more common in television and digital work, though a blue screen is better when the subject itself is green, such as plants.

Even light on the screen

The biggest challenge is lighting the screen evenly with no shadows, because the key works best when the colour range being removed is as narrow as possible. A shadow on the screen appears as a darker shade that the software may not remove. The material of the screen affects how easily it can be lit evenly, and wrinkles create shadows of their own. Shadows can occasionally be used on purpose, keyed to a darker version of the background so the person seems to cast a shadow into the new scene.

Spill and matching

A green screen bounces green light onto the subject, which is called spill. It can look unnatural and can make edges or parts of a person vanish in the key, so it is reduced in software or kept down by hanging a bigger screen well behind the actor. The light on the subject must also match the background scene they will be placed into. Overcast outdoor plates are easier to match because their light is soft and even; direct sun has to be matched in direction and colour.

Practice: a set-up that works

This is working practice. Light the screen with two or more soft sources from the sides, separately from the subject. Put the subject several metres in front of the screen to cut spill and shadow. Light the subject to match the planned background, and add a backlight to clean the edges. Use a fast shutter or minimise motion blur at the edges. Record in the best codec you have, ideally 4:2:2 or better, and avoid fine patterns and anything green on the subject. If the camera moves, add tracking markers to the screen.

Try it today

  1. 01Hang a green or blue cloth smooth and taut.
  2. 02Light it with two lamps from the sides and check it on a waveform.
  3. 03Place a person two or three metres in front with their own key light.
  4. 04Record ten seconds and try a key in your editor.
  5. 05Move the person closer and compare the spill on their shoulders.