Psychology -> Visual constancies
Perceptual constancies are learned visual perception principles which enable us to perceive stimuli which change in size, shape and brightness as remaining constant. Although the retina detects sensory changes, our familiarity with the observed object assists us in maintaining perceptual constancy.
Size Constancy: This is when we maintain an object's perceived size, even though the size of the retinal image alters due to changes in its distance from us. For example, when we look at a person from a distance of one metre, the retinal image is much larger than the one produced when looking at the person from a distance of two metres.
Nevertheless, we keep the person's size constant and do not think that the person is shrinking as they are walking away from us.
Shape Constancy: This is when we maintain an object's perceived shape, even though the image cast on the retina changes according to the angle at which it is observed. For example, the retinal image of a football alters depending on the angle at which it is viewed. However, we maintain the shape of the football and do not think its shape is distorting.
Brightness Constancy: This is when an object's perceived brightness, in relation to its surroundings, is maintained even though there are changes in the amount of light received by the retina. For example, the brightness of a car remains constant, even on a cloudy day with little sunlight, because everything surrounding it has been altered by the same level of brightness.
Orientation (position constancy): This is when we are able to maintain an object's location in space, relative to its surroundings. For example, light poles, trees and traffic lights are usually perceived as vertical regardless of which position we are viewing them. We don't think that the object has changed its orientation or location if we view it from a different position rather, we maintain its orientation as being constant by comparing it to its surroundings.