Lighting a green screen
How far from the backdrop to place the subject, and how to spread the light for a clean key.
The screen size nobody works out before buying
framed width (m) = sensor width (mm) × distance (m) ÷ focal length (mm)
It is the same triangle that governs every lens, and it settles the question exactly. A screen bought because it “looked big enough” is the most common expensive mistake in this whole area, because the shortfall only appears on the day, in front of everyone.
| Lens | at 2 m | at 3 m | at 4 m | at 5 m |
|---|---|---|---|---|
| 24 mm | 2.4 m | 3.6 m | 4.8 m | 6 m |
| 35 mm | 1.6 m | 2.5 m | 3.3 m | 4.1 m |
| 50 mm | 1.1 m | 1.7 m | 2.3 m | 2.9 m |
| 85 mm | 0.7 m | 1 m | 1.3 m | 1.7 m |
Read it the other way round when the room decides for you: a long lens needs far less screen, and stepping back with an 85 mm is often cheaper than buying three more meters of fabric. It also flatters faces, so the constraint costs nothing.
The four faults that no keyer will undo
Uneven screen. A keyer isolates one green. If the edges sit two stops under the center, you are asking it to isolate several. Light the screen with two soft sources crossing from the sides, aimed past the center rather than at it, and check the result on a waveform rather than by eye — the eye adapts to gradients and stops seeing them.
Spill. Green landing on the subject, and it is a distance problem before it is a software problem. Move them away from the screen, put a back light on hair and shoulders, and flag the sides. Spill suppression works by desaturating the edge, which takes skin tone with it — use it as the last step, not the first.
Shadows on the backdrop. The same distance solves it. If the room will not allow the distance, light the subject from higher up so their shadow falls on the floor rather than behind them.
Sharpening and heavy compression. An artificial edge is a fringe the keyer has to fight, and a codec that records color at quarter resolution leaves it very little to work with. Turn in-camera sharpening down, record the highest chroma sampling and bitrate you have. What each platform accepts.
Wrinkles, and what actually matters
A creased screen is the fault everyone worries about and it is rarely the one that ruins the shot — a wrinkle lit flatly and thrown out of focus disappears into a single green. What ruins the shot is a wrinkle catching a hard light from the side, because that creates a bright edge and a dark one: two greens again, in a shape that follows the fabric.
So the order of priorities is: even light first, distance second, flatness third. A slightly creased screen lit evenly keys better than a perfectly flat one with a hotspot in the middle.
Frequently asked
How big does my green screen need to be?
Exactly as wide as your lens frames at the distance you will be standing, plus a margin. The arithmetic is sensor width × distance ÷ focal length: a Super 35 camera with a 24 mm lens at 4 m frames 4.15 m, so a 4.8 m screen covers it with room to reframe. The calculator above does it for your camera. Nearly every screen bought too small was bought without doing this.
How far should the subject stand from the green screen?
At least 1.5 m, and further if the screen is wide. Two things go wrong when they stand close: green light bounces off the screen onto their shoulders and hair, and their own shadow falls on the backdrop. Distance fixes both at once, and it is free. This is workshop practice rather than a published standard — but the two faults it prevents are real and neither is fixable in post.
How evenly does the screen have to be lit?
Within about half a stop across its whole area — a third if you can manage it. Evenness matters far more than brightness: a keyer separates one green from everything else, and a screen that runs two stops darker at the edges is effectively two different greens. Two soft sources crossing from the sides, aimed past the center, beat one lamp pointed straight at it.
Should the screen be as bright as the subject?
Around the same exposure, or up to a stop under. Brighter than the subject and the green spills onto them and flares in the lens; much darker and the green gets noisy, which destroys the edge. The practical test is not a meter reading — it is to look at the screen on a waveform and check it is flat.
Why does my key have green edges?
Spill: green light landing on the subject. Three fixes, in order of how well they work — move them further from the screen, put a back light on their shoulders and hair from behind (it overrides the green with a clean edge), and add negative fill on the sides. Spill suppression in software is the fourth, and it works by desaturating the edge, which is why it costs you skin tone.
Does the codec matter for green screen?
More than the screen does, past a point. Keying works on color, and most delivery codecs record color at a quarter of the resolution of brightness — that is what 4:2:0 means. Shoot 4:2:2 or better if the camera offers it, at the highest bitrate available, and turn sharpening down: an artificially sharpened edge is a fringe the keyer has to fight.
Green or blue?
Green, by default: sensors have twice as many green photosites, so the green channel is the cleanest one you have. Blue when your subject wears green, has blonde hair lit warmly, or when the scene is night — blue spill reads as moonlight, while green spill never reads as anything but a mistake.