The eye uses three methods to determine distance:
- The size a known object has on your retina - If you have knowledge of the size of an object from previous experience, then your brain can gauge the distance based on the size of the object on the retina.
- Moving parallax - When you move your head from side to side, objects that are close to you move rapidly across your retina. However, objects that are far away move very little. In this way, your brain can tell roughly how far something is from you.
- Stereo vision - Each eye receives a different image of an object on its retina because each eye is about 2 inches apart. This is especially true when an object is close to your eyes. This is less useful when objects are far away because the images on the retina become more identical the farther they are from your eyes.