“We have never been a race-blind country, frankly,” Ms. Meister said. “I wish that I could say that the response would have been the same if everyone had been aware that she was Cherokee, but I don’t think that you can.”

Ms. Meister says she doesn’t think that Ms. Lange knew the details of Ms. Thompson’s background. Unlike with most of her other assignments, there are no known field notes from Ms. Lange about this shoot in Nipomo, Calif., and the captions in the Library of Congress are, Ms. Meister said, most likely inaccurate: Ms. Thompson’s relatives have insisted, for example, that they did not sell their tent for food as the captions declared.

With help from librarians at the San Francisco Public Library, Ms. Meister pieced together how the inaccurate caption information probably came about. After Ms. Lange filed her pictures to The San Francisco News, a reporter for United Press went to the migrant encampment in Nipomo. Although Ms. Lange — and, apparently, the Thompson family — had left days earlier, her photos were published with the United Press’s reporter’s article. Details from it appear in the captions in the Library of Congress, Ms. Meister said.

This edition of Ms. Meister’s book is part of “One on One,” a series in which each volume delves into a single piece in MoMA’s collection. The volume on “Migrant Mother” explores a print that was made for MoMA’s 1949 exhibit “Six Women Photographers.”

Which brings us to the case of the missing thumb.

It is easy to tell whether a print of “Migrant Mother” was made before 1939, because that year Ms. Lange had an assistant retouch the negative and remove Ms. Thompson’s thumb from the bottom right corner, much to the chagrin of Roy Stryker, her boss at the Farm Security Administration. While that was a fairly common practice at the time, Mr. Stryker thought it compromised the authenticity not just of the photo but also of his whole F.S.A. documentary project, Ms. Meister said. But Ms. Lange considered the thumb to be such a glaring defect that she apparently didn’t have a second thought about removing it. In the print from the Library of Congress below, the thumb is still in the image’s lower right corner.