Perception of distance.

[ebs2002]

Don't worry about the shape of the eye, instead think of the eye as a single point (if it makes it easier to picture, assume you're closing one eye).

Imagine an object 2 inches from your eye. Now draw a line from your eye to the top and bottom of that object. The angle is pretty big. Now move the object back an inch, and the angle is getting smaller.

The smaller the angle, the smaller the object.