How to use a green screen: shooting and keying tips

Why is a green screen green?

Because green is the colour least like skin, and the one cameras record most cleanly. Most camera sensors have twice as many green-sensitive pixels as red or blue ones, so the green channel carries the least noise, which makes the cleanest key. Blue works too and spills less, which helps with blonde hair or a green jumper, but it needs more light and is closer to the colour of denim and eyes.

Either way, the screen is only a tool for one thing: making the background a single, even colour that appears nowhere on the person.

How do you light a green screen?

Light the screen and the person as two separate jobs.

1. Light the screen evenly. Two soft lights, one each side at about 45 degrees, aimed across the cloth, give an even wash. The aim is one flat colour from corner to corner, with no hot spot in the middle and no dark edges. On a waveform the screen should be a thin, flat band.
2. Do not overexpose it. A screen blown out to near white loses its colour, and the key fails. Expose it a little below your subject's face.
3. Light the person for the new background. If the background plate is a sunlit street, the key light on the person should come from where the sun is in that shot, at the same warmth.
4. Add a back light. A light behind the person, on their hair and shoulders, separates them from the screen and fights the green that bounces onto them.
5. Kill the wrinkles. Stretch the cloth tight or use a painted wall or a pop-up screen. Each fold is a shadow, and each shadow is a slightly different green.

How far should you stand from a green screen?

At least two metres if the room allows it, more for a full-length shot. Distance does two jobs: the person's shadow falls on the floor instead of the screen, and less green light bounces back onto them. That bounced light is spill: a green fringe on hair, ears and shoulders that keys out with the screen and leaves a see-through edge.

Frame tightly so the screen fills the shot behind the person. Anything that is not screen, a light stand or the edge of the cloth, has to be cut out by hand with a garbage matte.

What should you wear in front of a green screen?

Anything without green in it, and nothing that reflects:

• No green or teal, even a little in a pattern: it keys out as holes.
• No shiny fabric, sequins or jewellery, which mirror the screen.
• Care with glasses, which can reflect green; angle the lights so the reflection misses the lenses.
• Solid, mid-tone colours key best. Fine, fuzzy fabrics and flyaway hair are the hardest edges to keep.

On the camera side, use the highest-quality recording setting you have, a lower ISO for less noise, and a faster shutter so fast hands are not smeared into the green.

Why is my green screen key messy?

Almost always because of the shoot, not the software. The usual faults, and where to fix them:

How do you key a green screen in Sharpy?

Sharpy keys like a compositor, in four passes: a garbage matte to cut away everything that is not screen, the core, the edge, and spill suppression, with choke to pull the edge in and light wrap to let the new background's light spill around the person. It judges each pixel by the direction of its colour rather than its brightness, so a shadow across the screen keys out with the lit cloth beside it. Tolerance and softness are yours to set, and when you leave them out, Sharpy says which values it used.

A keyed clip goes over whatever is on the track beneath it rather than replacing it, so you put the new background underneath: a still, a video, a colour or another shot. Sharpy's search covers stock video from Pexels and Pixabay at the size of your piece, and their credit goes into the video's description, never onto the picture.

Then clean the matte, the black-and-white map of what is kept, the way a grading page does:

• Open removes speckles and thin spurs.
• Close fills small holes.
• Grow and shrink move the edge out or in.
• Blur softens it.

Each distance is a share of the picture's shorter side, so a cleaned key means the same at any resolution.

You can also see what a key takes before you use it. Sharpy reports four numbers: the share of the picture the key takes, how many pieces it is in, how many of those are islands, and how noisy the matte is, with the raw key read beside a softened one so you see what cleaning would do. On a 4K frame of a test recording, a raw colour key on skin took 11.97% of the picture in 1,461 pieces, 1,432 of them islands. Softened, it took 11.55% in 141 pieces. Opened, closed and softened, 9.77% in 23 pieces, with 7 islands. Each reading takes 0.2 seconds on a 4K frame.

From Claude or Codex it is one sentence: "key the green, clean the edge, and put the city clip behind him."

Do you need a green screen at all?

Not always. Sharpy can find the person in every frame and cut them out with no green screen, which suits footage already shot in a room. A green screen still wins when the edge must be perfect: fine hair, fast hands, or anything see-through. Removing the background without a green screen covers the other route, and colour matching your clips helps the new background sit with the person.

Frequently asked questions

01

Can I use a green sheet as a green screen?

Yes, if it is matte, evenly coloured and pulled tight. Wrinkles and shine are the problems, not the material.

02

Is a green screen better than a blue screen?

For most video, yes: green needs less light and keys more cleanly. Use blue when the subject wears green or has very fine blonde hair.

03

How big should a green screen be?

Big enough to fill the frame behind the person at the distance you shoot from. For a seated talking head, a screen about two metres wide is usually enough.

04

Why does my keyed video look fake?

Usually the light. Match the direction and warmth of the light on the person to the new background, then grade the background to the person.

05

Does keying upload my footage?

Not with Sharpy. The key, the matte and the composite are all made on your Mac.