Photographer Dorothea Lange took some of the most powerful and iconic images of America during the Great Depression, and her photograph “Migrant Mother” is one of the most recognized and arresting images in the world. Yet few know the story, struggles, and profound body of work of the woman behind the camera. Dyanna Taylor, Dorothea’s granddaughter, talks about directing, writing, producing, and narrating the documentary “Dorothea Lange: Grab a Hunk of Lightning.” She’s joined by Elizabeth Partridge, Lange’s goddaughter, who is featured in it. “Dorothea Lange: Grab a Hunk of Lightning” is part of PBS’s American Masters series, and it premieres August 29, at 9 pm.