Much like the inside of the silo was shot in the interior of a practical silo set, the outside — specifically, in the scenes just before Regan and Marcus fall in, when they’re standing on top of the silo and lighting a fire — were also shot practically. The whole structure of the silo is real, as is the fire they light. But for reasons of safety, practicality, lighting, and sound, the filmmakers replicated the top of the silo inside an indoor studio.
We can see the process of shooting the scene in B-roll footage on YouTube; Millicent Simmonds and Noah Jupe walk around the practical silo roof while surrounded by grids and filming equipment. Then, of course, the studio environment around them was replaced in post-production by what we see in the movie: the night sky. Interestingly, the sky in that scene is just a big black mass with no stars. “In the beginning we were going to use greenscreen for the background, but we were running out of money because the number of VFX shots were growing,” cinematographer Charlotte Bruus Christensen told Filmmaker Magazine. “So we had to be creative and we just ended up doing a black space behind them. We hoped to put in a few stars in the sky [in post], but that never happened. So it was just a black background and a little wind machine. It was very simple. We lit it from the floor with lamps on stands.”
