Call me jaded, but I assume any image seen in advertising these days has had some ‘work’ done to it. If the Photoshop work is done well, it is almost like reading fiction, you can suspend disbelief long enough to imagine a world where no one has wrinkles and even super models can stand to have thinner thighs. Even at first glance you may not notice some tell tale signs of Photoshop work. Boing Boing in their post shared an article written by Scientific American that points out some ways to spot a Photoshop job. Also check out this page: Photo Tampering Throughout History.
Scientific American has a nice article listing the top five mistakes that photo-fakers make when they use photoshop to doctor piccies.
Surrounding lights reflect in eyes to form small white dots called specular highlights. The shape, color and location of these highlights tell us quite a bit about the lighting.
In 2006 a photo editor contacted me about a picture of American Idol stars that was scheduled for publication in his magazine (above). The specular highlights were quite different (insets).
The highlight position indicates where the light source is located (above left). As the direction to the light source (yellow arrow) moves from left to right, so do the specular highlights.
The highlights in the American Idol picture are so inconsistent that visual inspection is enough to infer the photograph has been doctored. Many cases, however, require a mathematical analysis. To determine light position precisely requires taking into account the shape of the eye and the relative orientation between the eye, camera and light. The orientation matters because eyes are not perfect spheres: the clear covering of the iris, or cornea, protrudes, which we model in software as a sphere whose center is offset from the center of the whites of the eye, or sclera (above right).