Dyanna Taylor had to look no farther than her own family to make a compelling film. After all, her grandmother’s photograph of Florence Thompson is seared into the American psyche as “Migrant Mother,” the Dust Bowl madonna enshrined by Dorothea Lange.

Less known was Lange’s husband, Paul Taylor, an economist devoted to studying farmworkers and, later, the impact of agribusiness. Ms. Taylor acknowledged that Lange was a much more charismatic and powerful personality than her gentle, quiet grandfather, but insists that none of Lange’s work would have happened without him.