“Finding Vivian Maier” opens Friday at Sundance Cinemas. Not rated, 1:24, three stars out of four.

Who was Vivian Maier? At the end of this lovely documentary, after hearing from the people who knew the suburban Chicago nanny best, we’re still not sure. And she likely would have wanted it that way.

What remains of her are the photos. In 2009, a Chicago writer named John Maloof was looking for historical photos of Chicago to illustrate a book he was writing, and spent $380 on several crates of old negatives at an auction. The photos didn’t really meet his need, but he discovered something else — they were really good. Most were street-scene portraits of Chicago residents from decades ago, lustrous black-and-white photos of matronly old women standing at street corners, drunks slouched on a front step, crying children being pulled along by their parents. (Her children are particularly good.)

Maloof was fascinated, and quickly bought up all of the other negatives at the auction. He knew he had a treasure trove of photography art that had never been developed, let alone seen. Who was this photographer, and why didn’t she ever put the talent that was so evident in her work on display?

“Finding Vivian Maier,” co-directed by Maloof and Charlie Siskel, follows Maloof’s efforts to find out who Maier was. In talking-heads interviews, we learn of a woman who was a social misfit, dressed as if she lived a couple of generations earlier. She comes across as private, eccentric, holding the world at arm’s length. Eventually, that eccentricity seems to metastasize into mental illness, and she ends her life known only as a strange old woman her neighbors see on a park bench, eating corned beef hash out of a can.

But if the portrait of the real Vivian Maier suggests a woman lost in her own world, her photos suggest a different person, one much more watchful and observant, even connected, to the humanity around her. Often Maloof and Siskel will cut from a talking-head interview about Maier to one of her photographs, showing a stranger, that somehow evokes the emotion of the moment. It’s as if she was expressing herself through her subjects.

In the end, after poring over old letters, interviewing everyone from the kids she nannied to Phil Donahue (who hired her when he was a Chicago-based broadcaster), we get a poignant if incomplete portrait of the woman. (The film is aptly called “Finding Vivian Maier” rather than “Found Vivian Maier”) There is a certain level of discomfort in the process — like Maloof, we are strangers sifting through another person’s life after they’ve gone, one who probably wouldn’t have welcomed the intrusion.

But we also circle back to the photos, which are now in high demand. And it’s hard to not to wonder how Vivian Maier’s life would have been different if she had stepped forward with her photos, been recognized for them in her time, rather than keeping them locked away in old boxes. If nothing else, “Finding Vivian Maier” urges us not to hide our own lights under a bushel.