Stephen Somerstein hasn’t seen “Selma” yet, but he doesn’t have to — he was there.

Over five fraught days in March 1965, Dr. Martin Luther King Jr. led hundreds of people from Selma to the Alabama State Capitol building in Montgomery in the dogged trek toward voting equality.

Somerstein, then a 24-year-old student at the City College of New York — and an editor of Main Events, one of its newspapers — decided to go, after marchers were foiled in two earlier attempts to register black voters, one ending in violence.

With five cameras and unfettered access, he shot more than 400 photos, 55 of which just went up at the New-York Historical Society.

The show, “Freedom Journey 1965,” captures not only the marchers but the hardscrabble landscape of that segregated time, right down to the ditches along the side of the road.

“I felt it profoundly ennobling,” says Somerstein, who piled into a bus with other CCNY students, many of them members of the Student Nonviolent Coordinating Committee (SNCC).

The bus dropped them off on the outskirts of Montgomery, where Somerstein followed the marchers to the Capitol. He says he was struck by how silent much of it was.

Though President Lyndon Johnson ordered soldiers to guard the marchers, the threat of violence hung in the air. Bystanders didn’t cheer, but looked on silently, their faces hopeful but wary.

For 30 heart-stopping seconds, Somerstein found himself on the stage directly behind Dr. King, seeing what King saw as he addressed the crowd, now 25,000 strong. Somerstein trained his lens on the back of King’s head and clicked the shutter.

The resulting photo was seen by the filmmakers, Somerstein says, who re-enacted that moment 26 minutes into “Selma.”