Do the Math: Planning Your Green Screen Shoot
Accurately plan your next green screen shoot with this technique.
Top image via Shutterstock.
Every so often, I have to shoot a group of people against a green screen to later composite with a background element. I want to share an easy way to “spec” out the screen size and stage space required to film this foreground element.
It’s simple, easy, accurate, and fun. In fact, this is how production designers used to lay out “stage plans” long before computer CAD programs were available.
The process relies primarily on the field of view of the lens used to shoot the background element. It’s essential that you shoot both the foreground and background with the same focal length lens. I always try to use the very same lens for each element. Sometimes this isn’t possible, particularly if the background was shot by a crew in a different country or if it’s stock footage you licensed. This happens more often than not, so you pray that their notes are accurate.
Our example today will be a VFX day exterior shot of seven people standing side by side — to be composited onto a background “plate” of a city street in Beijing. This article will only address the logistical and physical needs of the shoot, not the lighting or post-production processes.
Image via Panasonic.
The notes for the Beijing background element look good. They shot with a Lumix GH-4 camera in the 16×9 aspect ratio. The selected take was filmed with a 20mm lens. A reference shot was made with seven stand-ins composed to approximate the storyboard. This shot will be very useful for lighting and scale references. Here are the key measurements they recorded on location:
- Lens: 20mm
- Distance to Subject/Focus: 19’
- Camera Height: 54”
- Tilt: 0 degrees
We now have enough information to plan our shoot. The first thing I want to find out is how wide the green screen needs to be. The 3-foot tall platform for the actors will be 19 feet from the camera. The green screen will be set up 12 feet behind the center point of the platform
19 + 12 = 31. We now know that the screen will be 31 feet from the camera.
At that distance, how wide does the green screen need to be to fully cover all of the actors?
Calculating the Screen Width
First, we need to find out what the field of view of a 20mm lens on a GH-4 is — in degrees. I will get that information from the fabulous P-Cam app for IOS devices. This is an indispensable tool for all cinematographers, as it has technical data for every format of photography in use worldwide today. It’s been in my kit for years.
Step One — Field of View
It took me less than five minutes to find the horizontal field of view — in degrees — for a 20mm lens. That number was 46.78 degrees, which I rounded up to 47.
Step Two — Doing the Math
Now you’ll need a large sheet of graph paper, a protractor, ruler, X-ACTO knife, and a pencil. With the protractor, draw a 47-degree slice and carefully cut it out with the X-ACTO knife.
On the graph paper, make an x for the camera and then draw a 16-foot wide platform for the actors to stand on, 19 feet from the camera. Put a dot on the graph paper 31 feet from the camera to hold a place for the green screen. Then take the 47-degree slice and place the tip on the camera and align it vertically to the lines on the graph paper.
Trace the outline of the 47-degree slice. Then connect the view points at 31 feet from the camera. That equates to a 28-foot wide green screen.
Step Three — Lighting Allowance
Next, add whatever distance around the shooting zone you’ll need for lighting. That additional square footage will give you the minimum size shooting stage you’ll need to pull off this shot.
That’s all there is to it! Remember, this entire process depends on accurate notes and usable reference footage. I can’t overemphasize the importance of precise measuring because you might be shooting the background element for some other crew, so consider their needs for a successful outcome.
Do you know other ways to plan green screen shoots? Let us know in the comments.