Ebbinghaus Illusion, explained.
The Ebbinghaus illusion is a context-dependent difference in apparent circle size produced by surrounding circles of different sizes.
Why it happens
The left central radius stays at 20 drawing units. The right radius is adjustable from 12 to 28, while neighbouring radii differ. Removing the surrounds preserves the central circles.
The Ebbinghaus illusion is a context-dependent difference in apparent circle size produced by surrounding circles of different sizes.
Read the result
Separate apparent equality from the radius difference revealed after you remove the surrounds.
A worked example
Matching circles in context
Keep the left central circle fixed and adjust the right circle to an apparent match.
Record the match and remove the neighbouring circles while preserving both central radii.
Read the physical difference as your selected adjustment, with numeric cues and screen settings still uncontrolled.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
The left central radius stays at 20 drawing units. The right radius is adjustable from 12 to 28, while neighbouring radii differ. Removing the surrounds preserves the central circles.
Where this idea is useful
A practical use
When comparing dashboard marks, inspect their surrounding objects as well as the encoded values.
A common misconception
“The surrounding circles physically resize the centre.”
Only your slider changes a central radius. The surrounds change context, while the recorded dimensions remain inspectable.
What this explanation leaves out
- Numeric cues, screen geometry and self-adjustment prevent a calibrated estimate of illusion strength.
What if I choose exactly twenty?
An exact physical match is a valid response. One self-adjustment neither proves nor disproves a general perceptual effect.
Would the same chart mark look different in a more crowded panel?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.