Which spot is lighter?
Painters spend years learning to see value: how light or dark a patch really is, apart from everything around it. The eye is bad at this on purpose. It judges each patch against its neighbours.
You'll get ten pairs of spots in real paintings and pick the lighter one. Then the painting drops away and you see the two patches on plain grey. After that, a short test measures how far your eye drifts.
Takes about three minutes. Keys: A / B or ← / → to choose, space to continue.
Which ring holds the lighter value?
Which centre square is lighter?
How it's measured
Each value is CIE L*, computed from the image file's sRGB pixels: 0 is black, 100 is white, and a Munsell value step is about 10. Patches are picked automatically. The two patches in a pair differ by 4 to 14 L*, each is nearly flat inside, and their surroundings differ by at least 12 L*. A trap is a pair where the lighter patch sits on the lighter ground. Simultaneous contrast pushes it toward looking darker.
The square test runs two interleaved staircases. One grey sits on a near-black ground, the other on a near-white ground. The drift is where the staircases settle, minus 50. Your screen's brightness, gamma and the room light all change what you see, and ten pairs plus sixteen squares is a small sample. Treat the numbers as a demonstration, not a diagnosis.
Paintings: Caravaggio, The Calling of Saint Matthew (1599–1600). Vermeer, The Milkmaid (c. 1658). Alfred Stevens, In the Studio (1888). William-Adolphe Bouguereau, Breton Brother and Sister (1871). Hanabusa Itchō, c. 1710. Pietro Lorenzetti, The Crucifixion. All are public-domain reproductions.