The screen is not sized to the subject, it is sized to the plane it stands on
The lens projects a cone. At the subject the cone has one width, at the backdrop it has a larger one, and the ratio between them is simply the ratio of the two distances. Camera ten feet from the subject, screen eight feet behind: the backdrop plane is at eighteen feet, so the frame there is 1.8 times as wide. A 35 mm lens on full frame frames about 10.3 feet at ten feet, and about 18.5 feet at eighteen. The screen that has to cover the second number is the one you need to buy.
This is the arithmetic behind the most common complaint in small-studio chroma work, which is that the screen is somehow never wide enough. It is wide enough for the subject. It was never wide enough for the frame, and nothing about standing in the room looking at it tells you that, because your eye is not at the camera position and does not have the lens on it.
The consequence runs in both directions. Backing the camera up to fit more people in makes the problem better in one sense — the ratio between subject plane and screen plane shrinks as both get further away — and worse in another, because the absolute frame width at the screen grows. Going wider on the lens from the same spot makes it worse in both senses at once. Wide lenses and green screens are an expensive combination.
Separation is the whole argument
Every problem in a chroma setup is either spill, shadow or evenness, and separation is the lever on the first two. Green light bouncing off the screen lands on the back of the subject and tints hair and shoulders; the intensity of that bounce falls off with distance from the screen, so moving the subject forward is the direct fix. The subject shadow thrown by the key light lands somewhere, and if the screen is close it lands on the screen, where it becomes a second green that the key has to handle.
What separation costs is exactly the width computed above, and that is why the shape of the room matters more than its floor area. Twenty feet deep and twelve wide gives you plenty of separation and no screen width. Twelve deep and twenty wide gives you screen width and no separation. Given a choice, take the width — spill and shadow have partial workarounds through backlight and flagging, whereas running out of screen at the edge of frame has none except reframing.
Evenness beats brightness, and the seam is where it fails
| Symptom in the key | Usual cause on set |
|---|---|
| Edges hold but the middle tears | Hot centre, dark corners — a lighting evenness problem, not a key setting |
| A soft vertical line in the plate | A lapped seam catching light differently on each side |
| Green fringe on hair and shoulders | Spill from too little separation, or screen lights spraying past the screen |
| A dark band low in the frame | The floor-to-wall transition on a fabric backdrop, unlit and creased |
| The key needs different settings shot to shot | The screen level moved when the subject or the camera moved |
The single number worth measuring is the variation across the screen. Meter the four corners and the centre. A screen that holds within about a stop across its whole area keys cleanly at almost any setting; one that varies by two stops forces you to choose which part of the frame to serve. Adding a brighter light rarely fixes this and often makes it worse, because it adds another hot spot. Adding a light further away, or moving the existing ones back and feathering them across, usually does.
Fabric, paper and paint
Rolled material comes in fixed widths, so the width you need almost never matches what you can buy and you end up joining drops. The calculator laps them by whatever overlap you specify and counts the seams, because seam count is the number that predicts trouble. One drop is the ideal. Two drops with the seam behind a stationary subject is workable. Three or more, on a set that gets used regularly, is usually the point where painting a wall becomes cheaper than fighting the fabric.
Paint changes the arithmetic entirely: there is no seam, no wrinkle, and the coverage question becomes a paint-area question. The surface still has to be lit evenly and it still has to be big enough at the screen plane, which is what this page computes either way. A curved cove between wall and floor removes the horizon line, which is the other thing a fabric backdrop struggles with, and that is a construction question rather than a shooting one.
The margin field is doing more work than it looks
Fifteen percent sounds generous until the camera moves. A slow push in narrows the frame, which is fine. A pan of even a few degrees swings the frame sideways at the screen plane by a surprising distance, because the swing is multiplied by the full camera-to-screen distance rather than the camera-to-subject distance. At eighteen feet, five degrees of pan moves the frame centre about a foot and a half. If the camera is on a slider or a gimbal, work out the extreme positions and size for the widest of them rather than adding a percentage and hoping.
The same applies to a shot that will be stabilised in post. Stabilisation crops, but it also swings the crop window around, and a window that swings off the edge of the screen finds the wall. Shooting wider than delivery, which is the usual advice for stabilisation, means the screen must cover the wider frame — the field of view figures on the field of view calculator are the ones to use for that, entering the capture format rather than the delivery crop.
Questions people ask
How wide does a green screen need to be for one person?
It depends on the lens and the distances far more than on the person. The width to cover is the frame width at the screen plane, which is the camera-to-screen distance multiplied by the sensor width and divided by the focal length. A 35 mm lens on full frame, subject at ten feet, screen eight feet behind the subject, needs roughly eighteen and a half feet of screen before any margin. The same person shot on an 85 mm lens from the same spot needs about seven and a half feet. Nothing about the person changed.
How much separation between the subject and the screen?
More is better and the practical floor is around four to six feet for a single person, mainly so the key light shadow lands short of the screen and the green bounce off the screen stops tinting hair and shoulders. Beyond about ten feet the returns flatten and you are mostly paying in screen width. Where the room simply does not allow it, backlight and careful flagging recover some of the spill, and a screen that is only lit where the frame needs it recovers some of the shadow problem.
Is it better to buy a wider screen or use a longer lens?
A longer lens is usually cheaper and it also flatters faces, so it is the first thing to try. The limit is the room: a longer lens needs more distance for the same framing, and the camera has to go somewhere. Work it as a trade — the calculator reports the focal length that would fit the people in frame from where you are standing, and the distance that would fit them on the lens you have. Whichever of those two the room can supply is the answer, and if neither is available the screen has to get wider.
Do I need a green screen at all, or will any flat colour work?
A key needs a colour that is well separated from everything you want to keep, evenly lit, and reasonably saturated. Green and blue are conventional because skin tones sit far from both, with green usually favoured for digital sensors and blue where the subject wears green or where the shot is mostly dark. A flat wall painted a strong even green works as well as fabric and often better, because it has no wrinkles. What does not work is a colour that appears in the wardrobe, and that is a wardrobe decision made before the shoot rather than a fix afterwards.
How many lights does the screen itself need?
Enough to hold the whole covered width within about a stop, which is a coverage question rather than a wattage one. Enter the width a single one of your fixtures actually washes at the throw you use and the calculator divides. Two fixtures at opposite ends, feathered across so the far side of the screen gets the hot part of each beam, is the common small-studio arrangement and it covers more evenly than the same two fixtures aimed straight at the middle. Screen lights are separate from anything on the subject and the two should not be fighting each other.