As she read a biography of Cesar Chavez, the Mexican-American champion of farmworkers, the photographer Mimi Plumb remembered something. Actually, it was hundreds of things, beckoning from her attic: a trove of negatives from her days as an art student that she had tucked away in boxes long ago.

She had made the images in the Salinas Valley during the summer of 1975, when for the first time under a new California law, farmworkers were guaranteed the right to organize and choose union representation in secret ballots. Chavez’s 1,000-mile hike, known as the “caminata,” from the Mexican border to the agricultural heartland of the Central Valley, galvanized support for better wages and working conditions.

Paging through the book “The Crusades of Cesar Chavez,” by the journalist Miriam Pawel, Ms. Plumb, who had recently retired from San Jose State University, decided to finally go through her old work and opened the boxes.

The summer of 1975 came rushing back like a hot wind in the valley.

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Celestino Rivas, an organizer, shrouded in fog as workers listened. The Amezcua family marching together. Anonymous musicians carrying their instruments along a road. “I was like, ‘Oh, wow, look at this one,’ ” she recalled, using a stronger word than “wow.” She added, “I thought the work was better than I had thought at the time.”

Her images now form the basis of “Democracy in the Fields: The Summer of 1975,” a new website two years in the making that, under the auspices of California Humanities, serves as a multimedia album of photographs, audio clips and text telling the story of that summer through the people in the pictures. Ms. Pawel contributed research and helped to identify many of the people in Ms. Plumb’s images. Wendy Vissar, a photographer and web designer, pulled the elements together into a vivid, enduring chronicle and testimonial of the time.

“I had never really seen photos like hers,” Ms. Pawel said. “She had a different eye and a different focus and intuitively focused on the farmworkers as opposed to the leadership of the movement, and that made her photos really different.”

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Indeed, many of the images stand out. Ms. Plumb found one of Chavez she had never printed before. But what really drew her eye, the true treasures of the project, she said, were the scores of shots of the workers, many anonymous or their names lost to time, doing the painstaking work of persuading field workers, mostly impoverished Mexican immigrants or their descendants, to join the cause.

Ms. Plumb gained entree to their world through Bob Barber, a journalist who made introductions before she ventured out on her own. His recordings of interviews and rallies and meetings provide the site’s audio. Ms. Plumb, who grew up in Northern California suburbs and was a San Francisco Art Institute student at the time, did not speak Spanish, but through her youthful charm, she said, she broke down whatever barriers with her subjects.

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She used a small, discreet camera favored at the time, a Leica M2R with a 35-millimeter lens, sometimes a 50 for close-ups.

“I don’t let myself edit myself that much,” she said. “I took pictures very rapidly and kept my fingers crossed that I would get something. It’s a net that I threw out there. It’s not more thought out than that.”

Ms. Plumb quickly discerned the importance of the movement, but rather than photograph Chavez or the union offices, she felt drawn to the workers in the fields. Whether it was the light, the mood or the tableau, she pressed the shutter whenever a scene struck her.

“I worked very instinctively,” she said. “I would see things and think, Oh that’s interesting, that’s beautiful, that’s curious.”

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Ms. Pawel, who has written two books on the farmworker movement, said she was curious when Ms. Plumb called her and asked to show her some of the pictures. She was quickly won over when she saw the work. Aside from being a teaching tool, the images reinforce a broader message that should still echoes today.

“There is a resonance of the very simple, basic idea that people banding together can actually accomplish things,” Ms. Pawel said. “The amazing thing was that the poorest people in the state, not protected by any laws, could join together and overcome one of the most powerful industries in the state.”

It’s an idea that shines through in Ms. Plumb’s images.

“It was such a different world than what I lived in as an art student in San Francisco,” Ms. Plumb said. “I fell in love with the farmworkers, it’s that simple.”

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