Mick Hartley

The winning entry in the Best Visual Illusion of the Year Contest.

And the explanation:

Here is a novel illusion that is as striking as it is simple. The two images of the Leaning Tower of Pisa are
identical, yet one has the impression that the tower on the right leans more, as if photographed from a
different angle. The reason for this is because the visual system treats the two images as if part of a single
scene. Normally, if two adjacent towers rise at the same angle, their image outlines converge as they recede
from view due to perspective, and this is taken into account by the visual system. So when confronted with
two towers whose corresponding outlines are parallel, the visual system assumes they must be diverging as
they rise from view, and this is what we see. The illusion is not restricted to towers photographed from
below, but works well with other scenes, such as railway tracks receding into the distance. What this illusion
reveals is less to do with perspective, but how the visual system tends to treat two side-by-side images as if
part of the same scene. However hard we try to think of the two photographs of the Leaning Tower as
separate, albeit identical images of the same object, our visual system regards them as the ‘Twin Towers of
Pisa’, whose perspective can only be interpreted in terms of one tower leaning more than the other.