Eleni Karras took her first photograph of Athens in the spring of 1971, a boy on a bicycle on Ermou Street with a tank in the background, and her last in December 2021, from the window of the apartment in Kypseli where she died the following month at 79. In between she made, by the estimate of the archivists who have spent three years sorting her negatives, about 140,000 exposures of the same city.

Almost none of them were seen in her lifetime. Ms. Karras worked as a schoolteacher and later a court stenographer, photographed on weekends, and exhibited exactly four times, the last in 1998 in a gallery in Exarchia that closed within the year. On Thursday, the Benaki Museum opened “Eleni Karras: A Life in the Street,” a retrospective of 300 prints that its curators are calling the most important rediscovery in Greek photography in a generation.

“She was not forgotten, because she was never known,” said Dimitra Voulgari, the exhibition’s curator and head of the museum’s photographic archive. “What we have is a complete, continuous, deliberate record of a European capital across 50 years by a single eye. I cannot think of another one.”

The exhibition is arranged by decade, and the decades are legible on the walls. The 1970s are black and white and tense: shuttered shops, soldiers at bus stops, the crowds of the Polytechnic uprising in November 1973, photographed from a rooftop with a borrowed telephoto lens. The 1980s turn to color and to interiors, kitchens and kafeneia and the packed buses of a city that doubled in size. The 2010s, shot on a secondhand digital camera, return to the street, to queues outside banks and graffiti on Stadiou Street and an old man in Syntagma Square holding a sign that reads, in Greek, “I paid.”

Ms. Karras’s work came to the Benaki through her nephew, Yiannis Karras, a civil engineer, who found 212 boxes of negatives in her apartment and, not knowing what else to do, telephoned the museum. “I thought they would take a few boxes for the archive,” he said at the opening. “Three years later I am standing in a room with her name on the wall. She would have hated the attention and loved the room.”

The museum has digitized about 30,000 frames so far and plans to publish a book with a German photography house next year. Prints from the exhibition, in an edition of 10, are being offered through the museum at 2,400 euros each, with proceeds funding the remaining digitization. Ms. Voulgari said that several European institutions had already inquired about touring the show.

On opening night, a group of elderly women from Kypseli stood for some time in front of a 1984 photograph of a laundry line strung between two balconies, the Acropolis a smudge in the haze behind it. One of them, Vasiliki Antoniou, 81, said she thought the balcony on the left was hers. “I never saw her,” she said. “She must have been very quiet.”