Suddenly, it seems, the camera has become a potent weapon in what many see as the beginning of a new civil rights movement. It’s become a familiar tale: Increasingly, blacks won’t leave home without a camera, and, according to F.B.I. Director James B. Comey, more police officers are thinking twice about questioning minorities, for fear of having the resulting film footage go viral.

But the link between photography (or film) and civil rights dates back to Frederick Douglass, the famous …