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Here’s a tried and true tip that’s fun and still relevant.
Every so often we have to shoot a group of people against a green screen to be composited 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 use 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. 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 7 people standing side by side- to be composited onto a background “plate” of a city street in Beijing.
We will only be discussing the logistical and physical needs of the shoot and not the lighting or post production processes.
The following is my method for creating an accurate stage plan.
The notes for the Beijing background element look good. They shot with a Lumix GH-4 camera in the 16x9 aspect ratio. The selected take was filmed with a 20mm lens. A reference shot was made with 7 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’ tall platform for the actors will be 19’ from the camera. The green screen will be set up 12’ behind the center point of the platform
19 + 12 = 31. We now know that the screen will be 31’ from the camera.
At that distance, how wide does the green screen need to fully cover all of the actors?
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 indispensible 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.
F.O.V. 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.
Now you’ll need a large sheet of graph paper, a protractor, ruler, exacto knife and a pencil. With the protractor, draw a 47 degree slice and carefully cut it out with the exacto knife.
On the graph paper, make an x for the camera and then draw a 16’ wide platform for the actors to stand on 19’ from the camera. Put a dot on the graph paper 31’ 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’
from camera and that equates to a 28’ wide green screen.
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 accomplish this shot.
That’s all there is to it! Please remember, the entire process depends on accurate notes and usable reference footage. I can’t overstate the importance of precise measuring because you might be shooting the background element for some other crew, so be considerate of their needs for a successful outcome. Outcome meaning - a great looking two element composite.