How many metres of backdrop do you need for a cyclorama, a wall or a cloth to fill the frame? Too narrow and the edge shows. Too wide and you are lighting and renting for nothing. Here is how to find the right size, and how to get it in seconds with Cinecyclop.
Short answer. The visible backdrop width at a distance D equals (sensor width × D) ÷ focal length. On full frame with a 32 mm lens and the backdrop 6 m away: 36 × 6 ÷ 32 = 6.75 m wide, and 4.5 m tall using the 24 mm sensor height. The Cyclo tool in Cinecyclop works both out from the shot size and the subject to backdrop distance, rather than asking you for a raw distance.
What backdrop width depends on
The width of set the camera sees at a given distance comes down to the focal length of the lens, the width of the sensor and the distance between camera and backdrop. The shorter the focal length, the wider the field. The further away the backdrop, the bigger it has to be to fill the frame. Nothing else comes into it.
The formula
By similar triangles, the field width at a distance D is:
backdrop width = (sensor width × distance) ÷ focal length
Height works the same way, swapping sensor width for sensor height. Which is why you need the exact format of your sensor, Super 35 or full frame, and not just « 35 mm ».
A worked example
Full frame camera (36 × 24 mm sensor), 32 mm lens, backdrop placed 6 m from the camera:
- Width: 36 × 6 ÷ 32 = 6.75 m
- Height: 24 × 6 ÷ 32 = 4.5 m
So you need a backdrop of at least 6.75 m by 4.5 m to fill the frame. In practice you add margin, for camera movement and to be able to light the edges without seeing them.
Doing it in ten seconds with Cinecyclop
The Cyclo tool works out the minimum width and height of the backdrop. You give it the focal length and the sensor (the camera presets handle that), the subject height, the shot size you are after and the distance between subject and backdrop. The camera to subject distance falls out of the shot size on its own, which matches how you actually work: you know you want a full-length shot, not that you want to stand 4.20 m away.
The app shows the minimum size and lets you try several focal lengths in a row, which helps when you need the one that fits the stage you have. You stop redoing the maths every time the lens changes. The result stays next to the project, alongside the matching equipment list.
Frequently asked questions
How do you calculate the width of a backdrop so it fills the frame?
Field width at a distance D is sensor width × distance ÷ focal length. With a full frame sensor (36 mm wide), a 32 mm lens and the backdrop 6 m away, you get 36 × 6 ÷ 32 = 6.75 m. Height works the same way with the sensor height (24 mm), giving 4.5 m.
Why does focal length change the backdrop width needed?
The shorter the focal length, the wider the angle of view, so the more backdrop is visible at the same distance. A long lens narrows the field: the backdrop needed is smaller, but you need more room behind the camera. That is why you try several focal lengths before settling the size of the cyclorama.
Should you allow margin on the backdrop size?
Yes. On top of the theoretical figure you add margin for slight camera moves, for reframing in post, and above all to light the edges of the backdrop evenly without seeing where it ends. A few dozen centimetres each side is usually enough.
Does Cinecyclop calculate the backdrop height too?
Yes. The Cyclo tool works out minimum width and height from the focal length, the sensor, the subject height, the shot size and the subject to backdrop distance, with the camera height for placement. So it accounts for the real situation, not just the raw formula.
See also: Equipment list guide · Cinecyclop vs Camlist · About Cinecyclop.