Last weekend we had a chance to see “The B-Side,” Errol Morris’ wonderful documentary about the Cambridge portrait photographer Elsa Dorfman. I know Elsa through her husband, Harvey Silverglate, my friend and occasional collaborator. She also once took our family’s picture for a Boston Phoenix article. Our son, Tim, took Elsa’s photo a few years ago when he was attending photography school.

Dorfman is warm and outgoing, and her photos reflect that. Now mostly retired, she is best known for her work with a large-format Polaroid camera that takes 20-by-24-inch photos. And though she is known for her portraits of artists such as Allen Ginsberg and Bob Dylan, she’s also taken photos of literally hundreds of ordinary families who found their way to her studio. In the film, she comes across as intensely proud and self-aware, yet still the same person who once sold her photos out of a shopping cart in Harvard Square.

Here’s some backstory that the film does not explain: Several years ago Morris wrote a book about Dr. Jeffrey MacDonald, the former Army doctor serving a life prison term after being convicted of murdering his pregnant wife and young children. The book brought Morris into contact with Silverglate and Dorfman, as Silverglate is a member of MacDonald’s legal team. As Morris’ book, “A Wilderness of Error,” clearly shows, MacDonald did not receive a fair trial and may actually be innocent. (I reviewed the book for BookForum.)

Morris is a master storyteller, and Dorfman is an ideal subject. As Richard Brody wrote recently in The New Yorker, Dorfman is “a remarkable presence, a cinematic character whose comments distill a lifetime of wisdom, self-awareness, frustration, and survivor’s pride.” Go see it.