Perceptual Constancies- Size Constancy

Size constancy can be defined as the tendency to perceive the vertical size of two familiar objects to be the same, despite differences in their distance.

An example that makes size constancy more understandable is if you look a photo of a road or a tunnel. The road looks as if it is shrinking in the picture, but really it is the same size throughout the entire thing. The "smaller" part of the road is just further away, so it seems as if it is a different size then the rest of the road, but it actually is not.