Hi I’m Greg Johnson with Cinema Forte. I’m primarily a Producer now, but I started off as an editor and visual effects artist then moved on to being a DP,.. so I feel that I have a unique perspective on the entire workflow from pre-production through post on green screen projects.
I’ve developed a Best-Practice / rule of thumb for my team that I’d like to share with you in hopes it’ll help you achieve better results and have more fun.
If you find value in this presentation please subscribe to our social channels as we’ll be providing lots of additional tutorials that will further benefit the information you’ll receive here. Some topics include: Folder Structures & File Organization for Teams, Estimating & Budgeting Projects, Running a Profitable Business, and Intro to Client Management.
I personally hate long intros, yet here I am babeling on. Look - I’ve been pointing my camera at people and dinking around in After Effects for a really really long time and I’d love to share what I’ve learned and how I approach these types of projects as a whole. I’m not saying this is the right way - this is merely an overview on the way I do things -hopefully you’ll find a benefit in the following info.
- Lighting - Best Practice
Well we can geek out for days about the importance and relevance of lighting in cinema. Light is the fundamental component of getting an exposure, but beyond that it’s what we paint our subjects with and create a sense of space and mood. Lighting for green screen takes those principles and builds on it.
You’ll need to either be or hire a seasoned Director of Photography that can handle all the technical aspects of shooting on a chroma stage as well as keep the style and emotion of your project in check.
A good stage is important for audio, but having a chroma stage pre-light evenly with DMX or Luminair control can be a game changer when it comes to production day.
You’re going to need a lot of light to shoot correctly on a green screen stage for various reasons we’ll cover in a bit. But before we get into all that let’s make sure your camera doesn’t exceed its native ISO. Cranking the ISO beyond its native ISO will introduce noise to your image, and a noisy image is the enemy when it comes to pulling a decent key with detail.
Alright so we’ve set our cameras to its native ISO for us it’s ISO 800. Now we need to make sure the camera sensor is giving us the cleanist image possible - for that we’ll calibrate the camera sensor or “black balance” the camera. Before we proceed it’s important to get the camera up to operating temp. You want to mimic the conditions you’ll be shooting in, as well as the shutter speed and ISO before you engage the black balance. Once you’ve reached those conditions (about 15 minutes after you’ve powered on, and the camera is at room temperature) you should remove the lens and place the body cap on the camera. From here follow your manufacturer’s directions on how to Black Balance (ABB - Auto Black Balance) or Calibrate Sensor. Once this procedure is complete the camera body will remove any dead / errant pixels and the camera sensor will learn what true black is for your shooting circumstances - which is very important. If you don’t black balance who knows where the camera was last calibrated. Think of shooting with the white balance from the previous location - that would be absurd.
Now that our sensor is properly black balanced we can talk about white balance. In the past most cameras preferred shooting daylight (5600K) to reduce noise in the blue channel like at 3200k for keying. But nowadays camera sensors are pretty versatile. When I have complete control over lighting I almost always prefer shooting at 4500K. This allows tungsten to be warm, daylight to be blue and we are set smack in the middle of the color spectrum.
Most of the time when I do chroma key work it’s on an “epic” ad. So slow motion is my go to. When filming in slow motion I overcrank the camera in intervals of 24 (or the project timebase). If the project plays back at 24fps, then shooting in 24, 48, 72, 96, 120 fps etc is ideal. From there we can use simple math to truncate the over-cranked frames and playback at realtime if desired. Granted the shutter angle wouldn’t be at 180 degrees it would be less, but that’s actually a good thing.
A faster shutter speed means less motion blur,... as well as less light entering the camera. This is why I mentioned needing a lot of light on set - you’re going to need it.
Traditionally motion picture film is shot with a 180 degree shutter angle, or half the exposure of the frame rate. 180 degree at 24 fps = 1/48th shutter in standard photography. At 25 fps = 1/50th shutter, 30fps = 60th shutter etc.
The Filmic motion you’re accustomed to seeing in movies is based off a 180 degree shutter angle or 1/48th shutter speed. At this exposure there’s a fair amount of motion blur in a moving image. This is what makes motion pictures seem smooth between frames. If you bump up the shutter speed / lower the shutter angle then you get a stroby motion - think of the action scenes in Gladiator or Saving Private Ryan. When there’s an explosion and clumps of dirt go flying instead of seeing a blur you see the dirt clearly moving across the frame - albeit very stroby. Those action scenes were shot at a 45 degree shutter angle instead of the traditional 180 degree. 45 degrees of 360 angle is 1/8th. 8x 24 = 192. So A 45 shutter angle at 24fps = 1/192nd. At 30fps a 45 degree shutter = 1/240th.
When watching a film at home we want nice, smooth cinematic motion (not to be confused with the post-consumer crap “real motion” or “tru motion” where the TV turns films into a slurry soap opera mess).
When filming on green screen we want the strobist, fastest shutter speed we can get. I try to stay above a 1/90th shutter speed. Keying with motion blur is awful and never looks right - there’s no way to fix that. On the flip-side keying strobe\y motion is a breeze and I have a fix for that we’ll go over later. In short you want crips clean images to key, not motion blurred crap.
We’ll get into lighting for green screen soon, but one thing to keep in mind is using a strong backlight. When keying you want a clear separation from your subject to your key. With a nice touched by an angel halo around your subject the computer can easily identify where the rubber meets the road - I mean where the actor meets the green screen. This is an essential trick to cutting / separating your subject out cleanly from the background. Even if the backlight isn’t motivated you can choke the key to remove the backlight and still get a great key.
IRE is a unit of measurement to measure signals. In video, IRE roughly is 0 - black to 100 - white.
Sometimes you can show up on a production and shoot from the hip and get great results, but that’s a pretty wild approach. I prefer to storyboard and plan everything out with my team.
In order to pull off a realistic looking comp and scene you need to have the light in your world figured out, so that when you light on stage your lighting is motivated. Nothing says “something’s off” to your viewers brain then when the key on your actor is coming from the right, when the sun on your plate is coming from the left. It doesn’t make sense - and it looks very unnatural. So when you light, know your time of day that you’re emulating, know how practicals in your virtual world will require actual practicals on stage. For instance if there’s moonlight from behind, use a bright light at 6000K to be your moon. If your actors are standing by a fire, throw a light with a fire-effect to interact with the talent, then comp on fire after.
Know your contrast ratios and light accordingly,... if you’re shooting a scene to mimic broad daylight - light your talent harshly, use strong backlight and shadow. Stop down to f5.6 or f8.0 to mimic shooting mid-day. If your shooting a night scene light softly with little fall-off. Open up your aperture a little and get a shallow DoF (but be sure it’s not too shallow to key).
Storyboard the snot out of your scene. Know where the Sun or lighting sources are. Know where your shadows are going to be. If your talent is walking out of a shadow, then flag them off. This will help keep the lighting in your scene consistent. If the reverse shot is keyed from the same side as the master shot that doesn’t make sense.
Also give your talent a soul, don’t skimp out on an eye light. It’s hard enough to make a green screen comp look natural. If your talent doesn't have a glimmer in their eye, they look dead.
The number one issue that gives VFX artists heartburn when it comes time to key this crap, is green screen bleed, or contamination. Your lights don’t just light the green screen and stop, remember color is light bouncing off shit. So if you blast a few green walls and floor with light, it’s inevitable it’s going to bounce back onto your subject. It’s almost impossible to eliminate green screen bleed entirely, but we can mitigate the contamination quite a bit - which will make you a hero to your productions VFX artists, and keep your clients happy.
How to reduce bleed? Well for starters get your subject as far away from the green screen as possible. The further away from the green the exponentially better the bleed will be. Think of the inverse square law,.. But in a bad way.
The second biggest offender is the floor. When you’re on set you’re probably not looking at shadow detail, and noticing the green filling in under the chin and on clothes, arms, props, etc. It feels natural on set, but when you get into the edit suite you’ll see it alright. So throw down as much duvy or funries or anything you can to stop that green from coming back up.
The next step is to look at your subject and notice if there’s any green coming from the sides. Is there an adjacent wall off-camera that’s being lit unnecessarily? If so turn those lights off, or walk in a floppy to cut that bleed from spilling in. Also walk in as many floppies as possible on your negative side,.. The green has a way to bounce around everywhere and end up in shadows. Basically flag off everything you can and really really try your best from keeping any green from hitting your subject. You want your actor to essentially be isolated while all of the green to be far behind.
Oh remember a while back when I said to use a strong backlight to help cut your subject out from the green. Yeah still do that when you can. It makes a world of difference.
There’ll be times when you’ll have to roto out a flag, a stand or even a kids feet - trust me it’s easier to pull a great key and do a little roto than it is to pull a noisy crappy key then end up rotoscoping anyway.