1Two simple ideas
Your eyes look from two different spots
Your eyes sit a little apart on your face, so each one peeks at the world from its own spot. The closer a thing is, the more those two peeks disagree. Watch:
Two spots, two peeks
Each eye is in a different place, so each one sees the same thing from a slightly different angle. Hold a finger up and wink each eye — it jumps!
Close = bigger gap
The nearer the thing, the bigger the gap between the two peeks. Far away, both eyes see almost exactly the same thing.
2One eye vs two eyes
Two ways your brain can look
One flat picture. In our stripped-down picture, a near ball and a far ball can look the same size. With other depth clues hidden, one still view leaves the distance uncertain.
Two peeks that cross. The two lines aim in and meet on the ball. Comparing the views helps the brain estimate how far away it is.
3Your turn — move the ball
Watch the two peeks disagree
From above, here are your two eyes and a ball. Slide the ball nearer or farther and watch the two lines of sight swing — and watch how far apart the ball sits in each eye's little view.
4A new case: one moving eye
Can one moving eye recover a depth clue?
A camera has just one eye. Take one picture, slide it sideways, then take another while the balls stay still. Compare that with a camera that stays in one place. Size and overlap clues are hidden in this model.
Guess before you play
Can the two pictures from one camera at different positions give a clue to which ball is nearer?