In the 1860s, photographer William H. Mumler claimed he could produce portraits of people together with the spirits of their dead loved ones, few doubted his word. The country was in the throes of the spiritualist movement, when conjuring the dead through mediums and séances was a common affair. So, using a camera—a little-understood technology at the time—to do the same seemed just another tool of the spirit trade. Seeing was believing.

"People then understood that photographs were not always true," said Mia Fineman, a photo historian and curator of "Faking It: Manipulated Photography Before Photoshop," a new exhibition at the Metropolitan Museum of Art.

"Manipulated images have always shown up as a sort of footnote to the real story of photography, which is the story of unadulterated truth and immediacy," Ms. Fineman said.