How does the eye determine distance?

The eye uses three methods for depth perception or determining distance:

• The size a known object has on the retina - If you know the size of an object from previous experience, your brain gauges the distance based on the size of the object reflected on the retina.
• Moving parallax - Moving your head from side to side makes close objects move rapidly across your retina. Far away objects, however, move very little. This way, your brain can figure out roughly how far an object is from you.
• Stereo vision - Each of the eyes receive a unique image of an object on its individual retina. This is because each eye is about 2 inches apart. When an object is close to your eyes this is especially true. This is less useful for far away objects because the images on the retina become more similar the farther they are from your eyes.