The red bar near the top looks longer than the one near the bottom, but they are identical.
Try this
- Drag How much the rails converge toward 0 and the effect weakens.
- Tick Hide the rails and the bars match.
Why it happens
Converging lines are one of the strongest depth clues we have: think of railway tracks or a road meeting at the horizon. Your brain treats the top bar as farther away. Something that covers the same width on your retina but is farther away must be bigger in the real world, so your brain reports it as longer. This is size constancy, the system that stops people from seeming to shrink as they walk away from you, working in the wrong place.
Herschel Leibowitz and colleagues found the illusion was stronger in photos with richer depth cues, supporting the idea that it comes from our sense of space.
Did you know?
Mario Ponzo described it in 1911. Some studies suggest it is weaker for people who grew up in places with fewer long straight roads and buildings, though the evidence is debated.
References
- Leibowitz H., Brislin R., Perlmutrer L., Hennessy R. (1969). Ponzo Perspective Illusion as a Manifestation of Space Perception. Science, 166, 1174-1176. doi:10.1126/science.166.3909.1174
- Background on Wikipedia
The illusion on this page is drawn by code written for this site. Last checked 1 November 2026.
Frequently asked questions
Why does the top bar look longer?
The converging lines make it seem farther away, and the brain enlarges things it thinks are distant.
Try these next
Size and shapeMüller-Lyer illusion
Two lines of exactly the same length look different because of the fins at their ends.
Size and shapeEbbinghaus illusion
An orange circle looks smaller among big circles and bigger among small ones.
Size and shapeDelboeuf illusion
A disk inside a snug ring looks bigger than the same disk inside a wide ring.