Claude Lanzmann’s 1985 film “Shoah,” his best-known work, has been hailed as a masterpiece — one of the greatest films ever made. Publications such as Time Out and The Guardian deemed the film the “best documentary of all time.”

“Shoah” has also been derided for its 91/2-hour length: Influential film critic Pauline Kael once described it as “long and exhausting right from the start.”

Now, in “Claude Lanzmann: Spectres of the Shoah,” filmmaker Adam Benzine points his camera at the 90-year-old man behind “Shoah.” The 40-minute documentary, which premieres Monday, May 2 on HBO, looks back on the landmark Holocaust film and its maker.

Speaking in quiet tones with a matter-of-fact honesty, Lanzmann, the film’s lone interview subject, describes how he convinced Treblinka survivor Abraham Bomba to speak on camera about his experiences during the war. Bomba, who was then working as a barber in the Bronx, lost his wife and young son after the family was deported to the camp. Bomba’s life was spared due to his hair-cutting skills — he was forced to shave women’s heads before they were executed in the gas chambers.

Bomba was in his 60s when Lanzmann interviewed him —and fights tears as he tells the filmmaker that he cannot go through with the interview.

Now, decades later, Lanzmann recounts to Benzine how he convinced Bomba to see the interview through. It’s a heartbreaking sequence that underscores the deep wounds of Holocaust survivors, wounds that will never heal.

Lanzmann also recalls his determination to complete “Shoah,” even risking his personal safety to do so. He recounts a particularly harrowing day in which he attempted to interview a former SS officer with hidden cameras, and was forced to flee for his life when his subject realized what was going on.

Benzine allows his audience to catch a glimpse of Lanzmann the man. The French Jewish filmmaker speaks of his years fighting in the resistance, and of his father, who saved his life during the war.

Lanzmann also speaks fondly of his close friendships with literary figures and philosophers Jean Paul Sartre and Simone de Beauvoir. (Lanzmann and de Beauvior were in fact once romantically involved, and remained friends until the end of her life.)

Benzine packs his relatively short film with a treasure chest of material. We learn about Lanzmann’s personal life, as well as some details of his younger days as a resistance fighter, which appears to have fueled his desire to document the history of the Holocaust.

“Shoah” — the Hebrew word for annihilation — allowed survivors and witnesses of the Holocaust to share their memories. It preserved the legacy of history’s worst atrocity and preceded Steven Spielberg’s Holocaust Film and Video Archive by years.

Viewers of “Claude Lanzmann: Spectres of the Shoah” will come away with a deep understanding of Lanzmann’s intense passion for the project and why “Shoah” is such an important film.

Like Lanzmann before him, Benzine reminds us why the Holocaust must never be forgotten.

“Claude Lanzmann: Spectres of the Shoah” premieres at 9 p.m. Monday, May 2; also available for online and on-demand viewing.