Our misinterpretation of lines of the same length shows that the perceived size of an object does not depend on the size of the retina image alone, but that the distance of the observer from the object is significant. Vice versa, objects of known sizes are used to judge distances or to recognise the size of adjacent objects. From daily experience we know that this mechanism is sufficient to allow us to perceive objects and their size reliably. Therefore, a person seen a long way away is not perceived as a dwarf and a house on the horizon not as a small box. Only in extreme situations does our perception deceive us: looking out of an aeroplane, objects on the ground appear to be tiny; the viewing of objects that are considerably farther away, e.g. the moon, is much more difficult for us to handle.

In this case the perspective results in an optical illusion. The vertical line to the rear appears to be longer than the line of identical length in the foreground.