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It was a spot of blood on a mohel’s beard that first led Eliyahu Ungar-Sargon to question the whole business of circumcision.

At age 17, he was asked to hold a baby at a bris. That day, Ungar-Sargon witnessed up close the practice of metzitza ba peh, common in some Orthodox circles.

“He cut the baby,” Ungar-Sargon, now 32, recalled of the mohel. “The baby is screaming, and then [he] puts his mouth on the baby’s penis and sucks the wound. He had a dribble of blood on his beard. It got me thinking.”

So much so, the medical school student-turned-filmmaker eventually made a documentary about it: “Cut: Slicing Through the Myths of Circumcision.” The New Jersey filmmaker is currently on a cross-country tour screening his first feature — and sparking dialogue.

Ungar-Sargon will be on hand for an Oct. 29 screening and panel discussion at the Ninth Street Independent Film Center in San Francisco. The event is sponsored by Bay Area Intactivists, a local anti-circumcision group.

Scheduled to join Ungar-Sargon on the panel is Dr. Mark Reiss of San Francisco (executive vice president of Doctors Opposing Circumcision), Lisa Braver Moss of Piedmont (author of articles questioning the practice of circumcision and the novel “The Measure of His Grief”) and Rebecca Wald of Florida (founder of the Web forum Beyond the Bris).

“Cut” was completed in 2008, well before San Francisco became ground zero for the anti-circumcision movement, with a proposed ballot measure to criminalize the procedure. The measure was taken off the November ballot by the courts in July.

But the 70-minute film zeroes in on the same arguments heard locally from both sides at the height of that debate.

“I went into it with an open mind,” Ungar-Sargon said of the filmmaking process. “I was open to the information I was learning, trying to clarify and crystallize what I thought [about circumcision]. I had some serious questions before, but I did go in with a readiness to be changed by what I learned in either direction.”

In his case, the “intactivists” won.

Though he still strongly identifies as a Jew, he believes circumcision is “assault, an inconsistency in U.S. law when it comes to permanently altering the genitals of males and females. Infant boys deserve the same protection as girls.”

In his film, Ungar-Sargon interviews doctors, nurses, mohels, rabbis and prospective parents, all with a wide variety of views on the subject of circumcision. Many of his interviewees take a hard-line stance against it, calling it a violation of human rights.

An Orthodox man in the film, who appears without being silhouetted or digitally altered, calls the practice “abusive” and refers to himself as “an abuser.”

But Ungar-Sargon’s most fascinating interview is with his British-born father, Julian Ungar-Sargon, an observant Orthodox Jew, a neurologist and a believer in the importance of circumcision to Jewish identity.

He tells his filmmaker son it would cause him “great pain” if any grandsons are not circumcised. He also defends the practice, whether or not there are any purported health benefits.

“I thought it made for some dramatic content,” Ungar-Sargon said of casting his erudite father in the film. “My father has a great screen presence. With his British accent, it’s almost like talking to God.”

He added that he respects people like his physician father “who don’t quibble with me about the health benefits, but still do [circumcision] as religious Jews.”

When it comes to an outright ban, such as what was proposed in San Francisco, Ungar-Sargon knows of no good reason to oppose legislation that would make infant circumcision illegal.

But when reminded that the proposed San Francisco law could have sent parents and mohels to jail, he balked.

“It’s wiser to have a gradual falling off of this practice,” he said. “I do think jail time is too drastic a punishment for this practice at this point.”

He also had harsh words for a comic book by anti-circumcision activist Matthew Hess that was blasted for being anti-Semitic. The “Foreskin Man” comic included images that depicted mohels as snarling, fang-toothed monsters; also, Hess was the person who authored the language of the proposed law.

“There is a certain contingency of intactivists who I would characterize as negligently insensitive to other people’s cultures,” Ungar-Sargon said. “ ‘Foreskin Man’ is a very good example. I don’t know whether that person was motivated by anti-Semitism. I do know that most intactivists are not motivated by it.”

Ungar-Sargon does not think Jews will give up the practice of ritual circumcision anytime soon.

Rather, he hopes that over time Jews will discard the practice, much as ancient generations of Jews gave up slavery and capital punishment. After all, he says that accepting new ways of thinking is one of the things he likes best about Judaism.

“The part of Jewish tradition that makes me most proud is the dynamic ability to respond to new data,” he said. “Even if I respect the [Orthodox] position on [circumcision], that’s not the kind of Judaism I see going forward.”

“Cut,” 7 p.m. Oct. 29 at the Ninth Street Independent Film Center, 145 Ninth St., S.F. $6 suggested donation. www.bayareaintactivists.org.