CAMBRIDGE — A visitor stands in the lobby of the building on Massachusetts Avenue where Elsa Dorfman has her basement studio. “You here for Elsa?’’ a janitor asks. Say “Elsa’’ around Cambridge — or in the photography world — and people know who you mean.

Thanks to Errol Morris’s new documentary, “The B-Side: Elsa Dorfman’s Portrait Photography,’’ a lot more people are going to be on a first-name basis with her. It screens April 29 at the Brattle Theatre as part of the Independent Film Festival Boston. The documentary officially opens here on June 9.

“The B-Side’’ might be considered an early birthday present. Dorfman turns 80 on Wednesday.

The National Portrait Gallery, San Francisco Museum of Modern Art, and Museum of Fine Arts all own large-format Polaroid portraits by Dorfman. By the time she stopped taking them, about a year ago, she was getting $5,000 a sitting. That’s not bad for someone who during the early ’70s sold her black-and-white 35mm photographs from a grocery cart in Harvard Square, getting $2.50 a print. Dorfman cleared all of $700 during the 1973 holiday season. “Wow was it cold out there,’’ she recalls. “I loved coming home and counting the cash.’’

Dorfman’s married to someone else on a first-name basis with a lot of people he’s never met: criminal-defense lawyer and civil libertarian Harvey Silverglate. The couple have been Cambridge fixtures for nearly half a century. “Famous on Flagg Street,’’ Dorfman says with a laugh. Their son, Isaac, works in advertising in New York.

You don’t have to know Dorfman to know her, so to speak. Her moon face is almost as identifiable as her don’t-try-this-at-home fashion sense. Jumpers and running shoes? Of course. Polka dots and stripes? On occasion. More important, she has a personality as big as the Robbie-the-Robot-size Polaroid 20x24 camera that dominates her studio. Looking like something out of a steampunk fever dream, it’s one of only six ever produced.

Several photographers have made extensive use of the 20x24 Polaroid camera, including Mary Ellen Mark, Timothy Greenfield-Sanders, William Wegman, and Chuck Close. None has been as associated with it as Dorfman.

Perhaps the biggest factor in her first-name familiarity is, for lack of a better term, sheer Elsa-ness, a unique combination of ebullience, openness, and unconventionality. “It’s not so hard to find people who adore her,’’ says Morris, a longtime friend. He hails Dorfman’s “love of the spontaneous, her love of the imperfect, her involvement with life.’’

As Dorfman says in “The B-Side,’’ “I somehow have this misguided therapeutic idea that it’s my role in the universe to make people feel better.’’ The hundreds and hundreds of Polaroid portraits Dorfman took over more than three decades suggest that the idea might not be so misguided.

Like small-size Polaroid instant cameras, the 20x24 makes a unique print, developed within the camera and then extruded from it. The resulting image is phenomenally detailed, with notably full, rich colors.

Dorfman’s work has a distinctive look. She would use a white background, include white borders, and leave in roller lines (Dorfman calls them “tire marks’’), which are left by the rollers inside the camera that break the chemical pods used in developing the image.

Finally, using a steel-nib pen dipped in black India ink, Dorfman would write a caption on the bottom and sign the image. She started doing that with her photographs well before the Polaroid camera came along. Her work has always included text with image. That’s as it should be. Asked if she had youthful ambitions of being a photographer, Dorfman leans in and says sotto voce, “Not a photographer, a writer!’’ Much of her early work consisted of author portraits, particularly poets, and Allen Ginsberg was both mentor and close friend. In a real sense, Dorfman’s one of the last Beats.

Dorfman first used the 20x24 in 1980, for a dual portrait of Ginsberg and the poet Peter Orlovsky. It was a fitting start. Ginsberg and Orlovsky were partners, and Dorfman’s family portraits are her most celebrated work. As distinctive as the look of Dorfman’s big Polaroids is, their emotional fullness and warmth may be even more so.

“People would come to me because they really love their families,’’ Dorfman says. “That always shocked me. You know, Jewish families are very loving but they never say they love you? They’re like, ‘That was good — can’t you do better?’ So I’m surprised how much love there is. The awful thing is that I’ve had people who I know divorced, and I’ll look at their picture and think, ‘How can this be? How could this have happened?’ Then I feel like, ‘I have to help. I can’t let this go on: You can’t do that to my narrative!’’’ She laughs that Elsa laugh, knowing how ridiculous this sounds. The laugh doesn’t last as long as usual, though, since she’s partly serious.

The oldest of three sisters, Dorfman grew up in Roxbury and Newton and went to college at Tufts. How local is she? Dorfman still calls Government Center “Scollay Square,’’ doing so in an accent as thick as a slice of Boston cream pie. When a visitor admits to not reading The Boston Herald, she’s aghast. “You need to know what’s going on around here!’’

Local is not the same as parochial. Dorfman went to Brussels to work as a waitress at the 1958 World’s Fair. The photographer Weegee tried to pick her up. “I didn’t know who he was!’’ She lived in Paris, where she became friends with the writer Susan Sontag. (“Her French was so much better than mine’’). They lived in the same student hotel.

Moving to New York after graduation, she got a secretarial job at the publishing house Grove Press. That’s where she met Ginsberg, as well as the poets Robert Creeley, Charles Olson, and Robert Duncan, and the photographer Robert Frank.

Dorfman moved back to Boston, got a degree in elementary education from Boston College, and taught fifth grade in Concord for a year. One of the more amusing moments in “The B-Side’’ comes when Dorfman describes a sympathetic parent telling her, “You don’t really belong here.’’ Dorfman felt that way, too, and went to work for EDC (Education Development Center), in Waltham. Serendipity struck. She was handed a camera, a Hasselblad, and put to work in the darkroom (which had been set up by Berenice Abbott, no less). That was the first time she’d used a camera. She was 28.

Asked if anything in her background prepared her for photography, Dorfman gives a surprising answer. “I could never see myself in the mirror without wearing glasses. I was never in focus. I think that must have an effect on the veil between you and what you see. Also, you have to think — it’s really true what they say, you see with your brain, not your eyes — so I think the biggest thing was wearing glasses.’’

Morris and Dorfman have differing stories about the origins of “The B-Side.’’ He recalls an afternoon “many years ago’’ when he watched her going over old prints. “I thought, ‘This is a movie.’ ’’

Dorfman remembers the subject first coming up when she and Silverglate had dinner at Morris’s house in 2016. “This was, like, between ‘Pass the chili,’ ’’ Dorfman says, “‘I’m going to make a movie of you. Soon.’ ‘OK, Errol.’ ’’ Dorfman’s now rolling her eyes. “I figured it was like Errol saying, ‘I’m going to help you lose 50 pounds.’ It’s make believe. But then he called me a week or two later and said, ‘Well, I have the camera and the people and we’re going to start that movie.’ ’’

The title comes from Dorfman’s practice of offering her subjects a choice of two prints. She keeps the one they don’t take. She was thinking of doing a book of family portraits, but since the families have the official print, these would consist of the alternates. Dorfman mentioned this to a former subject in Amherst. “Oh, like the B-sides,’’ he said, the term used for the non-hit, or flip, side of 45 rpm recordings. She mentioned this to Morris, who fastened on it for the documentary.

“It’s the perfect title,’’ Dorfman says, “Even to this minute, I’m really a B-side person,’’ she laughs. When someone suggests she’s proud of that fact, Dorfman laughs even harder.

The 20x24 camera weighs close to 240 pounds. Dorfman once likened getting a print out of it to delivering a baby. Nafis Azad, director of photography for 20x24 Studio, which has the remaining film for the big cameras and operates several of them, equally marvels at Dorfman’s longevity and dexterity with the 20x24. “It’s amazing that she did it so long,’’ he says. “When we shoot in New York, there’s two of us running the camera. Elsa used to do it all by herself.’’

“I can’t really do it myself anymore,’’ Dorfman says. It’s only two blocks from Dorfman’s house to the studio, but she has a hard time walking the distance, because of failing kidneys. Shrugging, she describes herself as “on the edge of dialysis.’’ She doesn’t welcome the prospect; neither does she complain. “I guess I think, well, if I’m on dialysis I’ll just do a book about it.’’

It’s a very Dorfman attitude. This is a woman who ends every e-mail with “Onward.’’ If she neglects to include it, she’ll immediately send another one that does.

That word was Creeley’s standard farewell — they collaborated on a book, “En Famille,’’ in 1999 — and Dorfman, in tribute, adopted it after the poet’s death.

“I never thought I’d get to be so old,’’ Dorfman says, shaking her head. “There’s really something about age. When you’re young you think, ‘Oh, that’s malarkey.’ My mother used to take me to Filene’s Basement. ‘Oh, my feet are killing me,’ she’d say. I’d roll my eyes. ‘Oh, I want to see one more, one more, one more dress,’ I’d say. How she didn’t, like, pop me, I’ll never know.’’

Mark Feeney can be reached at mfeeney@globe.com.