A clean key is won at capture, not in software. Chroma keying removes one colour (usually green) and replaces it with a background. The keyer is only as good as what you feed it: an evenly lit green and a sharp, well-separated subject. Get the shoot right and the key is trivial; get it wrong and no amount of despill saves it. This is the workflow behind my ATEM Mini green-screen setup.
The one principle
Light the screen and the subject as two separate problems. An evenly lit green with good separation beats any software trick. Everything below serves that.
The green screen itself
- Matte, non-reflective fabric or paper. Shiny surfaces bounce hotspots.
- Wrinkle-free. Steam it or put it under tension. Every crease is a shadow and a different shade of green.
- Green vs blue. Green is brighter and needs less light, so it is the default. Switch to blue for blonde hair or green wardrobe.
- Do not wear green. Any green on you keys out too. (The trick behind Lieutenant Dan's missing legs in Forrest Gump: green tights.)
- Size it to fill the frame with margin so you never clip the edge of the screen.
Lighting is the real work
- Light the screen evenly. Two soft sources at roughly 45 degrees from each side, washing the screen uniformly. Aim for brightness within about half a stop corner to corner. No hotspots, no shadows. If the green is not a solid, even colour, the keyer cannot tell dark green from light green.
- Light the subject separately with your key and fill (my Elgato key lights). Screen lights and subject lights should not spill into each other.
- Add a hair or back light. A rim of light separates your edges from the green and is the difference between clean hair and a green halo.
- Match colour temperature. Keep all lights at the same white point (I run everything at 5600K daylight) so the green stays one consistent hue.
Separation and spill
Put 1.5 to 2.5 m between you and the screen. This single move solves two problems through the inverse-square law: your key light falls off before it reaches the screen (no shadows cast on the green), and the green light bouncing back onto you (spill) drops to almost nothing. Light hair and shoulders pick up spill worst, so distance plus a back light is what keeps edges clean. Standing right against the screen makes an even key almost impossible.
Camera settings for a clean key
| Setting | Target | Why |
|---|---|---|
| Aperture | ~f/4 | Sharp edges, lens at its best, screen still softened (see below) |
| Shutter | 1/100 to 1/125 | Cuts motion blur; blurred edges are semi-transparent and kill the key |
| ISO | Base / native | Noise becomes a crawling, ragged matte edge |
| White balance | Locked (custom/Kelvin) | Auto WB shifts the green hue frame to frame |
| Focus | Subject tack sharp | Soft edges cannot be cleanly separated from green |
| Codec | 4:2:2 if available, low compression | More colour detail at the edges to key against |
On shutter: a static talking head does not need the 180-degree rule (shutter about twice the frame rate). Bias faster, 1/100 or above, to keep edges crisp. My old note had this camera at 1/50, which is too slow; blurred edges are the first thing that breaks a key.
Why f/4 and not wide open
This is the part people get backwards, including my own earlier notes. The common advice is "open the aperture wide for a shallow depth of field so the green screen goes soft." Half right: a softer screen is good, but shooting wide open at f/1.4 also softens your edges, and that is exactly what wrecks the key. You want the subject sharp and the screen soft, and f/4 delivers both:
- Edge definition makes the matte. The keyer draws its cutout along your outline. Wide open, shallow depth of field softens the edges of your hair and shoulders. Those soft, semi-transparent pixels are a blend of you and the green behind you, and the keyer cannot cleanly assign them, so you get fringing and haloing. Stopping down to f/4 keeps your whole outline crisp and the matte edge clean.
- Lenses are sharpest around f/4 to f/5.6. Two to three stops down from wide open is the resolution and micro-contrast sweet spot, giving the keyer more real detail to work with.
- Enough depth of field for movement. At f/4 your face and body stay sharp if you lean in or shift. At f/1.4 a small move throws part of you soft, and soft is where keys fail.
- The screen still softens. With subject-to-screen distance, f/4 still throws the green slightly out of focus, which averages out wrinkles and uneven lighting into a smoother, more uniform green. You get the softened background without sacrificing subject sharpness.
Do not overcorrect the other way either. Past f/8 to f/11 the screen's texture and wrinkles snap back into sharp focus, and diffraction starts to soften the subject. f/4 is the balance point: subject sharp, screen smooth, lens at its best.
The keying stage
- Key in hardware or software. The Blackmagic ATEM Mini has a real-time chroma keyer (what I use); OBS and DaVinci Resolve do it in software with more control.
- Despill to strip green tint from the edges after the key.
- Feather the matte slightly and add a light wrap so the subject blends into the new background instead of looking pasted on.
- Do not over-key. Crushing the matte to kill every green pixel also eats fine hair.
Troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
| Green fringe on edges | Spill, no despill | More distance, add back light, apply despill |
| Patchy green, holes in key | Uneven screen lighting | Even out the wash, use softer/bigger sources |
| Jagged or torn edges | Soft focus, motion blur, too-shallow DOF | f/4, faster shutter, nail focus |
| Subject semi-transparent | Key too aggressive, or green in wardrobe | Ease the key, avoid green clothing |
| Shadow on the screen | Subject too close | Move the subject forward |
My setup applied
Collapsible green screen, evenly washed with two soft lights at 5600K; me about 2 m in front with the Elgato key lights; camera (the Sony a6700, or a Lumix S5 II) at f/4, 1/100, native ISO, locked white balance, outputting clean 1080p HDMI; key done in the ATEM Mini; the composite goes to Teams through the Cam Link. Camera choice and pipeline detail live in Cameras for home studio.
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