If depth perception works because the brain checks the difference in the position of the object between the two eyes and concludes how far away it is, how can we still see depth when one eye is closed?
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The short answer is: you can’t. But your brain can kind of fake it based on experience.
If you know how big a car is and then you look at a car, you can kind of know how far away it is based on its apparent size. But you don’t really *know* how far it is the way binocular vision would tell you. That’s good enough go not get hit by a car probably.
However, if I were to say toss you a ball, and it’s not a precise size you’re used to, if you’re trying to catch it you’re likely to struggle. You might catch it anyway or you might not, but it will be harder and you’re more likely to fumble if only using one eye.
Interesting test to give yourself. *CAN* you see depth with one eye closed? Cover one of your eyes and try to consistently touch the same spot on your phone or a desk top. Change the distance and see how often you are surprised when you contact the desk or where you touch. Try changing between moving your hand in from the side to touch the spot vs lining your hand up with your eye first and pushing out to touch the spot. Even when you are looking at something you remember very well at a distance you are used to it can be difficult to judge. And remember you brain does keep track subconsciously, if you really want to test it. Try doing this somewhere you aren’t familiar with 3d layout, and try not to survey with both eyes open first
And extension of this test is to find a place where you can see for miles. How well can you judge distance of objects more than 100 yards/meters away without using references? The further away an object is the less our depth vision works except in very general applications like “close” and “far”. It takes a trained eye and or decent references near an object to judge distance.
You may be able to infer general distance with one eye closed. Your brain is great at interpretation of color and shadow gradients to determine a 3d shape. However detailed depth can be very hard and if you close an eye before looking at something so you don’t have remembered depth data you might be surprised how hard it is to interpret what you are seeing for distance.
I think you’ll find you’ve got a slightly flawed question. Yes we can infer some limited distance with one eye, especially at longer ranges where our eyes are close enough together they don’t offer much parallax change (the difference in shape between two views offer of an object that we use to infer distance). But closer in. You do actually need both eyes for consistent and accurate depth vision, or more specifically, the accuracy needed becomes significant compared to what is generally needed with viewing distant objects. You can manufacture depth vision by moving you head around with one eye closed. But the depth awareness fades quickly when you stop moving your head.
Source: I’m a pilot and prior military. I’ve had a lot of training and practice with judging distances and speeds.
Because our brain has learned that certain appearances mean distance. Lines tend to bend toward a point in the distance (called the vanishing point) when something has depth, and you can see that with one eye. Shadows are also cues, and you can see lighter and darker colors with one eye. Your brain learns these patterns, so you can have a degree of depth perception without both eyes.
Because of focus. One eye still needs and can focus in and out, but it’s not fast enough for most eye-hand coordinaton activities. Hold a finger near your face, with one eye closed alternate looking at your finger and to whatever is behind it. It’s kind of hard, but you know what’s is far and near.
Basically one eye has depth-perception but doesn’t have the ability to “measure” it, you need two to be able to “triangulate” the focal point and focus fast on the object.
https://en.wikipedia.org/wiki/Depth_perception#Monocular_cues?wprov=sfla1
There are lots of ways we can judge depth with just one eye. Binocular cues are only part of the process.
A single eye focus on things close or far by [changing the shape of your cornea](https://www.aao.org/museum-education-healthy-vision/how-does-eye-focus). If the focal point is in front or behind our retina, the image will be out of focus and look blurry. That’s how we know if things are close or far, by focusing the lense of your eye and feeling how much.
How do none of the top answers mention this? It’s the main way we detect distance with one eye, all the other answers are secondary.
That kind of perspective is called ocular disparity and it’s only one kind of perspective. Others include:
Linear perspective, like you learn in art class
Retinal size: the closer something is the bigger it looks. So if you see two adult men about the same size , but one looks smaller, you subconsciously estimate distance based on that difference. Also works in coordination with linear perspective.
Overlay (I don’t actually remember the name of this one): closer things block the sight of things behind them.
Atmospheric perspective: blue and more neutral things look farther away. Think about how mountains look in the distance. Why they so grey-blue when if you get up close they’re bright green with trees? It’s because when youre far away you’re seeing the mountain through 12 miles of atmosphere, aka sky, and the sky is blue.
I was born blind in one eye and although I am no doctor I believe me eye uses size of shapes and shadows to determine distance. This lets me drive a car and be fine. However if you ever watch me try and catch a pop fly there is nothing in front or behind the ball for my brain to compare it to. I will run up and back many times try to adjust and it still feels like a guess. Sometimes I’m right most of the time I’m correcting at the last second when the ball is large enough to better judged the distance. That is in adult softball. Because there is no way in hell I could ever hit a baseball pitch. My 2 cents
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