An American collector has become an overnight multi-millionaire after a set of 65 glass negatives he picked up at a garage sale turned out to be genuine Ansel Adams works.

Rick Norsigian, a school district painter and collector, bought the photographic plates in Fresno, California for $US45 a decade ago.

They have now been authenticated and are valued at $US200 million ($223 million).

"The experts have emphatically determined that the negatives were in fact created by the iconic photographer Ansel Adams," said Norsigian, who released the findings on his website and at a press conference.

Mr Norsigian's lawyer Arnold Peter said the authentication presented a number of challenges, involving experts not only in photography but handwriting for notations and meteorology for evidence of weather at the time Adams took his famed landscape pictures.

"There is no definitive authority charged with authenticating photographs and unlike a painting there is no signature linking the work to the artist," Mr Peter said.

"So we decided to apply the highest possible evidentiary standard we could think of.

"Every individual who has actually examined all the evidence we have gathered has come to the same conclusion - these are in fact the works of Ansel Adams."

The specialists assembled by Mr Norsigian all back the claims.

Michael Nattenberg and Marcel Matley, two independent handwriting experts, said that handwriting on the envelopes in which the negatives were found belonged to Adams' wife, Virginia.

George Wright, a meteorological expert, concluded that one of the pictures found in the Norsigian negatives was taken on the same day and time as one of Adams' famous images.

Robert Moeller, former curator of the Boston Museum of Fine Arts, concluded: "After more than six months of close study, it is my opinion, within a high degree of probability, that the images under consideration were produced by Ansel Adams."

Art appraiser David Streets said he estimated the negatives' value at a minimum of $US200 million, based on current sales of Adams' prints and the potential for selling reproductions.

Adams, who died in 1984, was believed to have lost many of his negatives in 1937 in a darkroom fire.

The blaze destroyed as many as 5,000 negatives, or around one-third of his portfolio. Many of the negatives had never been developed into photographs.

Adams was one of the foremost nature photographers of his era, known for his images of the American west, especially Yosemite National Park.

Asked in an interview with CNN what he planned to do with the cash, Mr Norsigian said family and vacation would be high on the list.

"Well, number one, of course, I'm going to take care of my family, my grandkids," he said.

"And then I'm going to - my wife's been putting up with me for all these years and I want to get out in the summer and hopefully get to the coast."

- AFP