Although the new PBS documentary series The Vietnam War required more than a decade of production and clocks in at a healthy 18 hours, the filmmakers who made it are being criticized for not being thorough enough.

According to the directors Ken Burns and Lynn Novick, one of their biggest challenges was sifting through thousands of hours of archival footage, hundreds of thousands of photographs, and hundreds of hours of original interviews to piece together a coherent and compelling story. “I think when you tackle a complex narrative, I guess you presume that filmmaking is kind of additive,” Burns said at Asia Society on Tuesday. “And it is — you build a film together, but it's more often than not subtractive.”

“Though it's 18 hours long, it's still just a story,” he added. “And we've made decisions about that. It's not an encyclopedia.”

The series is told chronologically from France’s occupation of Vietnam in 1858 through the country’s 1975 reunification with newly unearthed archival footage and extensive interviews with subjects on all sides of the conflict. Burns said they benefited from making the film at this point in time because recently emerged scholarship has upended conventional wisdom about the war, and survivors in both the U.S. and Vietnam have become less reticent to talk about their experiences over the years. He noted that because of the complexity of viewpoints and perspectives the film depicts, he considers it “the documentary equivalent of a Russian novel.”

But some viewers, Burns says, have complained that it’s not complex enough. He recalled after one of the final screenings before the series’ premiere being pulled aside by a viewer who had been involved with the anti-war movement. They complained that, while the soldier experience had been covered in extensive detail, the anti-war movement hadn’t been done justice. Soon after, Burns said, he was approached by a former soldier who made the exact opposite complaint. “We tend to see things from our own biases,” Burns added. “Quite often, the thing that we want to be told in great detail isn't there.”

Burns and Novick recounted similar reactions on previous documentary projects they had done on the U.S. Civil War and World War II, in which they received criticism from across the political spectrum and accusations of political slants. Burns admitted though that he does harbor biases. “There's no such thing as objectivity — the only objectivity is coming from God and she's not telling,” he joked. “You always want to make sure that you are aware of the baggage that you bring or the particular bias that you might have, and we all have that.”

Burns recalled being personally affected by the Vietnam War — he was 11 years old when American ground troops entered the conflict in 1965 — and recounted his father’s strong opposition. Burns received a draft card when he was 18 in 1972 and mulled the possibility of claiming conscientious-objector status. (His draft number was high enough that he wasn’t ultimately drafted). He admitted being conflicted by the war at the time, but said that the media’s focus on his backstory has been “a bit overplayed.”

“None of that figured into anything that had to do with making this film,” he said. “In large measure because what we discovered was so startlingly new and upside down that it kind of made whoever you thought you were before kind of obsolete. It didn't matter.”

Novick noted that overall, the filmmakers have been pleased with the public’s reaction to the film — both in the United States and Vietnam. “We consider it the highest praise when we make an 18-hour film and are criticized for what we left out,” she said.

In the above video, Burns, Novick, and Thomas Vallely — a Vietnam War veteran and senior advisor on Southeast Asia at Harvard University's Ash Center for Democratic Governance and Innovation — discuss the documentary's reception in Vietnam. Watch the complete program — which also featured author Duong Van Mai Elliott — in the video below.