Leaning Towers

This illusion is very simple, but at the same time very striking. The picture shows two identical images of the Leaning Tower of Pisa. And yet the tower on the right seems to lean more. How does this illusion work?

It seems that the brain treats the two pictures as if they were part of a single scene. Normally, if you have two towers next to each other, they seem to converge as you look up, due to perspective. You get the same effect with the columns in a temple; here we see the Temple of Artemis at Jerash in Jordan, taken by Julianna Lindsay.

We know that the colums are parallel, although they look as if they are converging. But that is simply how parallel columns look to us, when viewed from a low angle.

Now the two pictures 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.

This illusion is based on the work of Professor Frederick Kingdom of McGill University, with whose permission it is reproduced here.

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