eli5: How does depth perception work?


I don’t naturally have it so visual explains don’t make sense to me

In: 6

Basically depth perception is your brain taking images from both eyes and bringing them together called “converging”. The depth of the object you look at is determined by small cues in each eye. For example if you put a pencil right in front of your face and moved it from side to side, you’d see it go out of vision from one eye and in vision of the other. Indicating it’s only a few inches from your face thus giving you depth perception.

Hold a finger up at arm’s length in front of you. Close one eye, and then the other, and notice how the finger appears to move against the background?

This effect is called *Parallax* and is fundamental to how we perceive depth. Because our eyes are not in exactly the same place, they see slightly different images of the world.

For depth perception, our brains take those two images and compare them. Over time, we’ve learnt to recognise that the closer an item is, the greater the difference in its apparent position to each eye, and thus approximate the depth of an item.

You can also ask a friend to focus looking at the tip of your finger, and watch their eyes as you move it closer and further away from them. You’ll be able to see their pupils move closer together as your finger gets closer to them. This is much the same idea: when looking at an object, if our eyes need to be closer together to merge the images then the object is closer to us.

Naturally, this all requires two eyes. There *is* a third factor that only needs one, involving recognising that closer items appear larger (cue Father Ted: [these ones are small, those are far away](https://youtu.be/MMiKyfd6hA0)), but this relies heavily on recognising shapes as physical objects with a known size. This is still enough to trip people up if something appears to be an object much larger or smaller than it really is.

Two images from very slightly different angles gives the brain enough information to gauge depth a little

You mentioned visual explanations aren’t very helpful for you, but, here is one anyway

If you look at a cube straight on, with no depth perception, it appears to just be a square (one side). If you look at it from slightly to the left, you can see some of the left side. Same if you look from slightly to the right. If you combine both of these images, you actually see a little of the left and a little of the right at the same time, allowing the brain to form a better idea of the actual shape (a cube).

You do have depth perception, or you wouldn’t be able to do anything. You wouldn’t be able to even walk around in the world, because you would have literally no idea how far away from you anything was.

What you may lack, and what a substantial minority of the population lacks or is impaired in (I think about 5-10%), is stereovision, which relies on the combination of the images from your two eyes. The most common cause of this is when one eye is highly dominant over the other, or one eye has impaired vision.

The main binocular depth cue is binocular parallax (AKA binocular disparity or stereopsis), which others in this thread have explained. You are lacking access to this particular source of depth information. However, there are many others, including motion parallax, perspective, relative size, and occlusion, which don’t depend on having two functioning eyes working together. These *monocular* depth cues allow you to function mostly normally.

You (likely) mostly notice that you don’t have good stereovision when doing things that require precise depth perception, and/or when monocular cues are missing or not precise enough. For instance, catching a ball or threading a needle. And of course, you also notice when going to watch a “3D movie” (i.e. a movie presented as two images to your separate eyes, with binocular parallax), or when trying to see the 3D image hidden in one of those “magic eye” pictures. But it’s a common misconception that that’s all that depth perception is, and you (and others) are living proof of that, since you aren’t confined to your bed and being fed through a tube.

one interesting thing about depth perception is that sometimes, when you see the moon rising, it is near the “floor” on the horizon, perhaps near some buildings or trees. Your brain thinks “wait a second, the moon there near those builds? It cannot be that small”. So the brain enlarges the moon for you. You see a giant moon. You take a picture and here it is the moon small as ever.

So, sometimes you are beeing fooled by your brain, that can render things instantly, at hight “framerates” and mix them seamsly into the “frame stream”.