When Errol Morris announced to photographer Elsa Dorfman, his friend of more than two decades, that he wanted to make a movie about her, Dorfman rolled her eyes. She knows the Oscar-winning filmmaker is always juggling a few projects at once and probably wouldn’t find the time.

So it was a bit of a surprise when, four days later, Morris said he was ready to go.

“I said, ‘What? Right now?’ ’’ says Dorfman.

That was last week. Since then, Morris, a documentary filmmaker whose credits include “The Thin Blue Line’’ and “The Fog of War,’’ has spent three days with the 78-year-old photographer famous for her large-format Polaroids.

“I’m a little surprised I hadn’t done it before, actually,’’ said Morris, who, like Dorfman, lives in Cambridge. “It’s about time.’’

The time is indeed right because Dorfman is about to stop taking photos — not because she wants to, but because the film and chemicals she needs to make her pictures are in exceedingly short supply. Dorfman figures she can keep working for just another few weeks.

“I have very little left and the film is not such good quality,’’ she says. “Every batch is touch and go. It’s kind of like using old cake batter.’’

Last week, Morris was fortunate to have his camera rolling as movers from Gentle Giant took some of Dorfman’s oversize portraits out of the house. (She’s having digital copies made of the 40-by-80-inch prints.)

“I came back this week and we spent the time mostly in Elsa’s garage talking about her photos,’’ he said.

Over the years, most of Dorfman’s subjects have been ordinary people — couples, families, etc. — but she’s also photographed a few of her famous friends and acquaintances, including Julia Child, Allen Ginsberg, Faye Dunaway, Jonathan Richman, and, of course, her husband, celebrated defense lawyer Harvey Silverglate.

“I describe Elsa’s work as the perfect combination of Renaissance portraiture and dime-store photography,’’ says Morris.

Dorfman said she’s thrilled that Morris is making a movie, but sad that her career as a photographer is coming to an end.

“It’s very traumatic,’’ she says. “This is like my identity. It’s what I do.’’