Why do objects appear as flat/“lost a dimension” when observed from a great distance away?
In: Physics
Our two eyes being a few centimeters apart gives us two slightly different 2D pictures which allows the brain to calculate the 3D shape from that difference. At large distances that difference becomes so minuscule that you only get the same 2D image twice and thus there is no 3D shape to calculate.
Depth is perceived in the brain as a function of the ratio of the distance from the eyes to the closest part of the object to the distance from the eyes to the farthest part of the object.
If you have a person 2 meters away sticking their hands out directly at you, the hands may be only 1 meter away.
The ratio 2:1 on distance means that the person is twice as far away from you as their hands, and your brain perceives a large depth.
Now move the person to 101 meters away and their hands are now 100 meters away. The person’s body is now only 1% farther away from you as compared to their hands rather than twice as far, so you perceive less depth.
>Simple answer: We never had to be able to do that for survival.
>Longer answer
Our eyes are designed to see the world in 3D: they are constantly adjusting and soaking up data to tell your brain what’s going on around you. This is why your eyes strain after looking at your TV for too long: your eyes are trying to picture a world in 3D…on a 2D surface.
Evolution tells us that the human eye found no reason to make eyes complex enough to see stuff that’s really far away. This is why hawks and eagles have excellent eyesight.
There are a few answers already posted that go into some of the mechanics of the eyes, parallax and depth perception, but I’d like to take a slightly different approach.
It’s almost easier to think of it in the other way around: Why do objects appear “not flat” when you are close up. Bear with me, I’m really going for ELI5 here.
You are right, things do appear flat at a distant. But consider this: Our eyes work a bit like a camera, capturing light into flat (2D) images. On their own, all of these images appear flat, which begs the question, why do things ever appear not flat?
Well, it’s because what we “see” isn’t just a stream of info from our eyes. Our brains do lots of work in the background to add information from a bunch of different sources to help us get a better idea of what we are looking at. Part of this process is adding in depth information, so we get the sense of the “not flatness” of the things.
There are lots of sources that our brains use for this. Some are famous, some are not so famous. What’s really fun is that there are ways to “trick” each of these information sources, which is where we get optical illusions. Some of them off the top of my head, and how you might have seen them used against you in film making:
* our two eyes see slightly different things – this is how 3D films make things look “not flat”, they show you two different images to either eye and our brain adds some info for you to see
* we are good at comparing to things we’ve seen before – if you’ve ever seen The Lord of the Rings, when you see the Hobbits pick up a “pint glass” and it makes them look tiny.
* we know how things are “supposed” to move – that’s why we need to speed up special effects done with miniatures. Our brains would see the movement and add the wrong info for the scale
* perception of the mechanics of light and shadows – this one is really hard to trick, which is often what makes CGI look really flat or out of place in movies. Our brains will even chance the colour of something based on this. Remember the blue/black or white/gold dress?
* Focus range – if our eyes can’t keep everything in focus, our brains will add info on depth from that. Easiest to see in fancy photos. If you blur the background, everything in the foreground sticks out.
This isn’t close to a full list. Also, none of these are vital for building an “not flat” image of the world, but each plays it’s own part.
So why is there a difference between up close and far off? Well, because most of the information sources that work for our brain to add in that depth info don’t work at very long distances. Both eyes will see identical things, so that’s no good. At a big enough distances it’s really difficult to compare size and movement of objects, so that doesn’t work. Everything is a practically identical focal length, so no good there either.
So the TL:DR of the ELI5: Everything we see is flat at first, but our brains have ways of figure out depth for objects that are up close to make them appear “not flat”
When the object is close, your left and right eye have a difference in image that you can notice by switching between them
When it is far away, the difference is less noticeable.
Because of this, your brain is like “Ah that’s 2d.”
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