Christopher Nolan’s 70mm WWII spectacle Dunkirk has already received more attention and critical praise than almost any other film this year, and it’s only hitting theaters today.

We’ve read and written a lot about what unique spin Nolan might bring to a well-trod topic, and of course about the tech behind the film. It's one of the first features in history shot almost entirely with a million dollar IMAX camera. (This crazy Business Insider story reveals how the team rescued footage from one that sank unexpectedly during a water-landing scene.)

But why was it so important for Nolan and his DP Hoyte Van Hoytema (Interstellar, Her) to shoot on the rarely used 70mm format? Was it more than just a gimmick? This behind-the-scenes video reveals some of the team’s decision-making process.

Van Hoytema says that the team considered 70mm from the very beginning. “We straight away started talking about a big format, especially IMAX. It’s a very visceral window to the world...In many ways, it's the best format out there.”

Producer Emma Thomas adds, “If somebody had said to us six years ago, ‘Go off and make a film about the evacuation of Dunkirk using only large format cameras, I think we would have thought that was an insane thing to do.” However, since they had some experience with the format between Interstellar and the Dark Knight movies, “we were equipped to do it and were able to maximize the format and really get everything that we wanted from it.”

That being said, Nolan admits that “a huge challenge is the size and weight of the cameras, so our feeling was, if we could find a way to do it physically, the payoff would be well worth it.”

In the video, Van Hoytema and Thomas detail the lenses and other accessories that they had to develop to make the camera usable in terms of weight and movement. Akin to the Dunkirk soldiers depicted in the film, Van Hoytema recalls, “You always end up thinking about things that are impossible and then try to make them possible.”