Appearance vs. Reality

The checkershadow illusion is one of the more remarkable "illusions" I've encountered, that goes to the heart of how we (as visual information processors) interpret and categorize the world; it also goes to the heart of the question, "Are you really sure of what you are looking at?"

Believe it or not, the squares marked A and B are exactly the same shade of gray! If you do not immediately believe your senses (as I suspect you won't!), just copy/save the image to a jpeg file and use any image processing program to sample the actual luminosity of each square...truly amazing!).

The "explanation" is that our visual systems require more than just luminosity to assess the shade of grey to be assigned; it also needs such features as local contrast and boundary effects. A complete explanation is provided here.

The checkshadow illusion was devised by Edward H. Adelson, Professor of Vision Science in the Dept. of Brain and Cognitive Sciences, Massachusetts Institute of Technology. His site includes many other startling illusions that explore the nature of perception and interpretation of reality (see his Illusions and Demos), as well as technical papers explaining his theories and findings. Adelson's site is a must-see for all photographers who "believe" they know all there is to know about appearance, reality, and the true nature of tonal gradations.

Additional references (and illusions) appear on the Perceptual Sciences Group homepage.