Leaning Tower Illusion

What is happening

Two identical panels of a tall tower seen from the ground are placed side by side. The right tower should look like it leans farther to the right, with the two towers seeming to drift apart as they rise. But the two panels are simply the same image shifted sideways — not a single degree of angle or shape has changed. Revealing the answer draws each tower’s center line in the same color to show they are parallel.

Why it looks that way

A building seen from below appears to narrow toward a single point as it rises. Your visual system tries to merge the two panels into “two towers standing in the same scene.” If two truly parallel towers were seen from below, their images should converge toward each other near the top. Since they don’t converge in this figure, your visual system interprets it as “the towers are actually splaying outward,” which you feel as a difference in lean. Frederick Kingdom and colleagues reported this in 2007, and confirmed it also happens with photos of roads or railway tracks, not just towers.

How to check

Cover the gap between the two panels with your hand and look at each one alone — the difference in lean nearly disappears. That it only happens when they’re placed together is evidence you are reading them as one combined scene. Even after seeing the answer lines, going back usually still looks tilted.

See an illusion where perspective cues change apparent size

First described

After Frederick Kingdom, 2007
Source: Kingdom, Yoonessi & Gheorghiu, "The Leaning Tower illusion: a new illusion of perspective", Perception (2007)

Published with Ali Yoonessi and Elena Gheorghiu. Winner of the Best Illusion of the Year Contest that same year.

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