A film that’s become more urgent than ever

In exchange, Sonderkommandos received slightly larger living quarters, books, better food, alcohol and cigarettes. After four months, they were slaughtered themselves and replaced with other prisoners, who were then sadistically made to burn the bodies of their predecessors.

Nemes became fascinated with this group after stumbling upon a volume of testimonies written by the Sonderkommando titled Voices From Beneath the Ashes. Eventually, he and co-writer Clara Royer devised a story about a man who believes that a boy who survived the gas chamber, only to then be murdered by a Nazi doctor, is his son, and sets out on a personal mission to give the dead child a Jewish burial.

Using a bold aesthethic, Nemes keeps the camera focused mainly on Saul's expressionless face, or sometimes follows him from behind, immersing the viewer in his subjective experience. The periphery of the frame is often blurred, leaving us to imagine many of the horrors happening there. It is a powerfully effective technique, and creates a way of visualising the Holocaust that has even won the praise of Claude Lanzmann.

The director of the landmark documentary Shoah once decried attempts to recreate the Holocaust on film, saying: "There is a certain degree of horror that cannot be transmitted. To claim it is possible to do so is to be guilty of the most serious transgression." However, Son of Saul has won him over. "It's very original, very unusual. It's a film that gives a real sense of what it was like to be in the Sonderkommando," he said. "It's not all melodramatic. It's done with a very great modesty."

Inevitably, the film has reopened the historical debate about whether the Sonderkommando were themselves perpetrators. When a reporter suggested that they were "half-victims, half-hangmen" at the film's press conference in Cannes, Rohrig angrily leapt to their defence.

"They are 100 per cent victims," he said. "They have not spilled blood or been involved in any sort of killing. They were inducted on arrival under the threat of death. They had no control of their destinies. They were as victimised as any other prisoners in Auschwitz."

They are nonetheless a difficult subject and it is perhaps telling that Nemes struggled to raise funding for the project. The lion's share eventually came from the Hungarian National Film Fund, prompting Elod Novak, deputy chairman of the far-right Jobbik party, to write a Facebook post saying his party would "put an end to the Holocaust industry in filmmaking".

In fact surging antisemitism and an increase in Holocaust denial (disingenuously termed revisionism) on the internet make the need for a film like Son of Saul, which persuasively connects the dots in the industrial process off transforming hundreds of humans at a time from flesh, blood and bone into ash, more urgent than ever.

Its embrace by critics has been almost unanimous. It performed well at the box office in Hungary, although not as well as Nemes would have liked in a country responsible for sending most of its Jews to their deaths, and now looks set to vie for one of cinema's highest accolades. Not bad for a film that refuses to see the Holocaust as anything other than it was: genocide.

Son of Saul opens on April 29