Leaning Tower Illusion

The leaning tower illusion is a visual illusion seen in a pair of two entirely identical photographs of the Leaning Tower of Pisa, taken from below and placed side by side, in which the tower on the right appears to lean noticeably more steeply than the tower on the left, as though the two photographs had been taken from different vantage points, even though the two images are pixel-for-pixel identical copies of one another. The vision scientists Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu, working at McGill University, discovered the illusion, which won first prize in the 2007 Best Illusion of the Year Contest. The researchers attribute the effect to how the visual system ordinarily corrects for the perspective distortion produced by a tall object photographed from below, in which the object's sides would normally appear to converge toward the top; because the two identical towers placed side by side actually have outlines running parallel to one another rather than converging, the visual system, which treats the pair of images as a single unified scene rather than as two separate photographs, perceives the parallel outlines as diverging instead, producing the appearance that the two towers lean apart from each other.