Ebbinghaus illusion

Same disc, different neighbors.

Measured from the image

Sliders

How the effect works

Two orange discs sit side by side. Large gray discs ring one of them. Small gray discs ring the other. The disc among the large neighbors looks smaller. The disc among the small neighbors looks larger. Both orange discs have the same radius.

The build script counts the orange pixels inside each disc. The two counts match, so the discs agree in area as well as in radius. Only the surroundings differ. Your visual system judges size by comparison. A circle beside big shapes reads as small, and a circle beside small shapes reads as large.

The sweep table raises the radius of the big flankers while the small flankers stay put. The proxy climbs as the size contrast between the two rings grows. At the low end of the table the two rings look alike, and the discs look alike with them. Change the gap between each center disc and its flankers and see how the effect responds for you. Change the number of flankers too. The orange discs keep their size through every setting, and the pixel counts show the match.

Measured at the default settings

Image size: 552 by 276 px. The build script read these values from the rendered pixels.

Left Center Radius Px: 28
Right Center Radius Px: 28
Left Center: x 138, y 138
Right Center: x 414, y 138
Left Center Area Px: 2453
Right Center Area Px: 2453

Where the effect weakens

The table sweeps Radius of the big flanker discs (px) and scores a geometric proxy from 0 to 1: Flanker size contrast (big - small) / (big + small), full at 0.6. The proxy comes from the image geometry. The proxy measures geometry and not perception. Your own eyes decide what you see. Rows below 0.5 are marked weak.

Weak range: 20 px. Proxy at the default setting: 0.977.

bigRadius (px)	Proxy	Left Center Area Px	Right Center Area Px	Centers Equal
20	0.417 (weak)	2453	2453	true
30	0.714	2453	2453	true
40	0.897	2453	2453	true
50	1	2453	2453	true
60	1	2453	2453	true
70	1	2453	2453	true
80	1	2453	2453	true

Try this

Set the big flanker radius close to the small one. Watch the two discs look alike.
Move the flankers closer, then farther away. Note how the effect responds for you.
Change the number of flankers around each center and compare.

History

The illusion is named for Hermann Ebbinghaus and is also called Titchener circles. Sources disagree on the year, so the site gives no date. Source

Only facts confirmed against a source appear here. Where sources disagree, this site leaves the detail out.

More illusions

Entertainment only. Not a vision or health test.