It is the ability of the brain to keep in focus. It deals with stability in changing environment. If constancy principle is not at work, the world becomes a chaotic and disorganized for the individual. Without perceptual constancy, the shapes, sizes, colours of the objects would change as a person moves about; and this makes his job uncomfortable and sometimes impossible. The essence of perceptual constancy is that the shape, size, colour, brightness, and location of an object reasonably and fairly constant regardless of the information recived through various sensory organs.
Perceptual constancy, also called object constancy, or constancy phenomenon, the tendency of animals and humans to see familiar objects as having standard shape, size, colour, or location regardless of changes in the angle of perspective, distance, or lighting. The impression tends to conform to the object as it is or is assumed to be, rather than to the actual stimulus. Perceptual constancy is responsible for the ability to identify objects under various conditions, which seem to be “taken into account” during a process of mental reconstitution of the known image. For example, snow appears white in the low illumination of moonlight, as well as in sunlight 800,000 times as bright. Perceptual constancy is reduced by limited experience with the object and by decreasing the number of environmental cues that aid in identification of the object.