In the late 19th century, photographers accidentally stumbled upon the quirk of double exposures. This could happen as a result of long exposures which made figures appear ghostly. "If the subject moved during the exposure, they appeared in the finished photograph as a blurred, transparent, ghost-like figure." It could also happen if a glass plate or piece of film was inadvertently used twice, as there was no foolproof way of determining whether or not a glass plate or sheet of film in a holder had been exposed.
The unintentional soon made way for a new market: spirit photographs. Initially, "ghost photos" were sold as novelty items with no claim to being of a supernatural nature, and books were published that provided instructions on how to create them, but it didn't take long for unscrupulous individuals to see the financial potential in exploiting the grief stricken who longed to believe that their deceased loved ones were nearby.
William H. Mumler in the 1860s was the first to propagate spirit photography. After accidentally creating a double exposure, he began marketing himself as a medium, doctoring photos of people in mourning by adding images of their dead relatives. Mumler was most likely inserting a plate featuring the departed relative into the camera
His shenanigans came to an end when he put the faces of people who were still alive and well in Boston into the photos.
The most infamous of the spirit photographers was William Hope. He led a group of fellow tricksters called the Crewe Circle. In the wake of World War I, their services were in great demand by those who'd lost relatives in the conflict.
In 1922, Harry Price from the Society for Psychical Research investigated Hope and his Circle and found they were "substituting glass plates bearing ghostly images." In his published research, Price noted, “Mr William Hope has been found guilty of deliberately substituting his own plates for those of a sitter” and that “It implies that the medium brings to the sitting a duplicate slide and faked plates for fraudulent purposes.” Despite exposing Hope as a con artist, there were still people who were desperate to believe his photos were authentic. Among his most devoted supporters was Sherlock Holmes author Sir Arthur Conan Doyle.
Of course, double exposures weren't only deployed for nefarious motivations. Some photographers used them for startling artistic effect, as in this photo of saxophonist King Guion.
Today, many high-end digital cameras allow intentional multiple exposures. Photoshop enables users to produce multiple exposure images by manipulating the layers of a digital image file.
And even in the age of digital camera phones, modern counterparts to spirit photographs can be accidentally attained due to a process known as 'image aliasing.’ Phones sometimes take photos in stages. This can be a slow process and "anything moving through the shot at the time could appear distorted."
This occurred when a schoolgirl's photo on a trip to Hampton Court Palace appeared to show a sinister apparition. Those eager to believe in ghosts said it was the Grey Lady haunting the palace. The actual explanation was that "As the woman moves from the right to the left, she is captured at various parts of her journey – making her hair look very tall, while her body has moved away."
So while it's easy to mock those who fell for Mumler and Hope's scams, it appears that even in the 21st century, there are still some who forego common sense in favor of believing a technical quirk is a sign from the beyond.
I ain't afraid of no ghost!