Before Nov. 22, 1963, Abraham Zapruder was an ordinary citizen of Dallas: a 58-year-old prosperous manufacturer of women’s clothing who had arrived in the city from Russia by way of Brooklyn. If that day had unfolded differently, that is most likely what he would have remained. But like a small but growing number of Americans at the time, he was also a home-movie hobbyist. With his receptionist, Marilyn Sitzman, as location scout and technical support, Zapruder took his 414PD Director Series Bell and Howell 8-millimeter camera to a spot on Elm Street, not far from his office, hoping to film the presidential motorcade as it drove past.

The 26.6 seconds of footage he captured — 486 frames, without sound — inscribed Zapruder’s name in the official history and popular folklore of the John F. Kennedy assassination. Its images, blurry yet vivid, in color when almost all television and a great many movies were still in black and white, form part of what we know or think we know about what happened at Dealey Plaza. On the Web, you can find seemingly infinite versions: in slow motion, with musical accompaniment, with or without Kevin Costner’s explanation and Oliver Stone’s enhancements in “JFK.” And you can encounter an equal number of arguments about what those silent, shaky frames mean: that Oswald acted alone; that other shooters were present; that a conspiracy came to fruition in plain sight; that the truth will never be known....