Perceptual constancy
Objects look stable even as their retinal image changes.
What it means
Perceptual constancy is the tendency to perceive an object's stable properties — its size, shape, color, and lightness — as unchanging despite large variation in the proximal sensory input caused by distance, angle, and illumination. The visual system achieves this by combining the raw image with contextual cues, effectively discounting viewing conditions to recover the object's enduring characteristics. Size constancy uses distance cues so a person seen far away is not perceived as tiny; color and lightness constancy discount the illuminant so a banana looks yellow under varied light. Constancies are largely adaptive and automatic, but the same discounting machinery produces striking illusions when context misleads. They reveal that stable perception of a stable world is itself an inferential achievement.
Examples
A door is perceived as a rigid rectangle even though, as it swings open, its retinal image is a changing trapezoid.
A white shirt looks white under warm kitchen bulbs and under blue midday sky, even though the light bouncing off it is a completely different color in each case.
A friend walking away down the street does not seem to shrink, though the image on your retina halves — hold up a thumb and you can see how small they actually are.
First described in Studied since Helmholtz (19th c.); central to Gestalt and vision science.