If you’re curious about your ability to spot the difference between real images and those that have been Photoshopped, Adobe has put together a little online quiz. When I took it, I guessed correctly 15 out of 25 times, or about 60% of the time.

“More often than not, people think that the real images are fake and that things that are fake are real,” says Farid. “And their confidence is very high. So people are both ignorant and confident, which is the worst combination.”

The solution is to turn to computers to spot the inconsistencies that humans can miss. Photographic forensics uses a battery of techniques and algorithms to identify fake photos, many of which examine whether images fit with the laws of physics. While it may never be possible to authenticate a photograph to 100% confidence, forensics specialists can test photos using a number of techniques.

Let’s look at a famous photograph that has had conspiracy theorists whispering for decades.

This picture of Lee Harvey Oswald has often been claimed by conspiracy theorists to be a fake (Credit: Warren Commission/Wikipedia Commons)

The photo above is of Lee Harvey Oswald, the former US Marine who assassinated President John F Kennedy in 1963. According to authorities, the photo was taken in Oswald’s backyard and sent to his friend in April 1963. Investigators used it as evidence of Oswald’s guilt after matching markings from the rifle in the image to the gun found in the Texas School Book Depository in Dallas, Texas, after the assassination. Questions over the authenticity of the photo have fueled conspiracy theories that Oswald was framed for the assassination by the government, or criminal groups, particularly because Oswald himself denied the photo was real and was killed by a gunman before he could stand trial.

Conspiracy theorists have pointed to a few features in the picture as “evidence” of tampering – the shadows, particularly those on Oswald’s face, appear to some as if they are cast from a different light source than the shadows of other objects in the photo. Oswald’s chin looks broader than in his mugshot while his stance supposedly looks odd given the weight of the gun while others dispute the length of the gun itself in the image.

Farid and his colleagues examined the photo in a series of papers published in 2009, 2010 and 2015. In their analyses, researchers built 3D models of the scene and of Oswald based on his mugshot, his known height and weight, and the weight of the gun. They found that the shadows in the scene were consistent with a single light source, with the shadows on his face accounting for the appearance of a broader chin than that in his mug shot.

They also found that his posture was plausible given his centre of mass and the way he was carrying the gun, and estimated that the length of the rifle in the photo, after accounting for perspective, was 40.186in (101.2cm) long, less than an inch shorter than the length reported by the manufacturer. In all, the researchers couldn’t find any evidence of photo tampering.