Size constancy

Size constancy is how we perceive objects as having a constant size, even when we increase or decrease how far away we are from them. An example of this would be of this picture where the boy appears a lot smaller than the man that is closer up. Your brain however knows that the boy is much bigger than he appears because we perceive him as having a constant size.

http://farm5.static.flickr.com/4154/4831420198_0312c079b2.jpg