The phone call was routine, the kind often made before big auctions. Sotheby’s was preparing to sell a striking rust-brown image of a leaf on paper, long thought to have been made by William Henry Fox Talbot, one of the inventors of photography. So the auction house contacted a Baltimore historian considered to be the world’s leading Talbot expert and asked if he could grace the sale’s catalog with any interesting scholarly details about the print — known as a photogenic drawing, a crude precursor to the photograph.
“I got back to them and said, ‘Well, the first thing I would say is that this was not made by Talbot,’ ” the historian, Larry J. Schaaf, recalled in a recent interview.

“That was not what they were expecting to hear, to say the least.”

In the weeks since Dr. Schaaf’s surprising pronouncement was made public, “The Leaf,” originally thought to have been made around 1839 or later, has become the talk of the photo-historical world. The speculation about its origins became so intense that Sotheby’s and the print’s owners decided earlier this month to postpone its auction, so that researchers could begin delving into whether the image may be, in fact, one of the oldest photographic images in existence, dating to the 1790s.