A fundamental phenomenon of visual perception is the idea of constancies. The mind and brain perceive issues as constant, despite things changing in everyday life. How is this possible? The mind perceives some constancies: shape constancy, color constancy, and brightness constancy.

Shape constancy is one type of perceptual constancy. The critical aspect here is that the mind interprets an object's shape as constant even though its shape changes on the retina and in real life. Therefore, even if a coin is presented at an angle that causes it to appear as an oval on the retina, it is still perceived as a circle.

Another perceptual constancy is the tendency to judge objects as the same size no matter how far away they are from the viewer. So, if a 5-foot-tall item seems very small on the retina, it will be considered quite far away.

Lightness constancy is the third type of perceptual constancy, which is the tendency to perceive an object's perceived lightness as constant even when the light conditions change. If a person wears black pants and a white shirt, the shirt will appear significantly lighter in bright daylight than the pants. However, if the sun is obscured by dense clouds, even though the pants and shirt have less light to reflect, the shirt will appear as bright as before because lightness constancy is at play.