How do our eyes/brains perceive depth?


For example take a tree. How can our brains tell that it is a big tree far away instead of a smaller tree which is just closer to us?

In: Biology

Your brain gets two images, one from each eye and uses that information to triangulate depth more or less. There are different degrees of depth perception: first your brain has a simultaneous perception of an image coming from both eyes, your brain then overlaps those images so that you don’t see double, with that overlap comes the actual sense of depth. However, it is a highly manipulable sense. You can talk yourself into seeing all sorts of oddities very easily.

Here’s an experiment: hold up a single pencil and look at a blank wall behind it. You should notice you see two pencils. If you don’t, blink a lot and hold the pencil a little close or further from your nose. Grab a second pencil, hold it about an inch or two away from the first pencil and you should see four pencils while looking at a wall between (behind) them. If you merge the middle two pencils you just created into 3, you will perceive the third pencil in the middle slightly behind the pencils on either side of it. There’s a real life demo for you, right there!

Your brain measures the slight difference in angle between your eyes looking at the same object. As the distance gets farther, the difference becomes smaller and your judgment of distance becomes less accurate. Context and memory also help.

Before I got glasses as a kid, I used to gauge distance by degree of fuzziness, too. Your brain is more clever than you are.

It is learned as well as instinctive.
First,our brain compares the sizes with something you know of close to you.(comparing a pencil with your hand fro example)

Second is by relative intensity. Something close will give off more light than something far away.

Third, there’s a thing called parallax,which is shift in position of object when you look with one eye closed. Objects close to you shift more than those far away. When riding a car,objects far away don’t zoom past that quickly(say a mountain or building) compared to a bystander on sidewalk.

Up close, your brain uses trigonometry – it uses the difference between the images coming into each eye to calculate distance.

At greater distances, where the angle is too small, it uses your knowledge of how big things should be. (Which is why it is very hard to estimate distance if there are no familiar objects in sight – a car, building, tree, person, etc).

Long time ago my coworker told me a cool story about her friend who was born with some genetic issue that took away his ability to perceive depth. His whole life was 2d until he was 21, he tried LSD with some friends and for the first time in his life he saw the world in 3d.