(Image: David Bachrach/Library of Congress)
For a long time, it was thought that there was only one existing photograph of President Lincoln at the dedication of the Soldiers’ National Cemetery at Gettysburg on November 19th, 1863. As you can see, it's a shot of the dignitaries on stage, taken by David Bachrach (interactive graphic #3 identifies many of the people in the picture).
But Bachrach wasn't the only photographer at the event. Alexander Gardner and his crew were there, and made an experimental "stereograph" photo of the crowd. A picture was taken from two slightly different vantage points, with the aim of showing them together through the left and right eye for a 3D effect. With the technology of the time, they could have been taken several minutes apart. In 2007, John Richter identified Lincoln in the background of the stereograph photos. Now there were three photographs of Lincoln at Gettysburg!
(Image: Alexander Gardner/Library of Congress)
Then enter UNC-Asheville media teacher and former Disney animator Chris Oakley. A longtime Civil War buff, Oakley took a new look at Gardner's crowd photographs. He and his students have spent the past two years analyzing the pictures and have found Lincoln, but in a different spot. To back his claim, the team used computer feature analysis of both the "new" Lincoln and the people around him to make identifications.
Of course, to proclaim such a find, Oakley had to have the data to back it up. Oakley's quest to confirm one person in a large crowd shot is detailed at Smithsonian magazine, with an interactive look at the photographs. Link