How can our eyes tell the difference between a close object and a far object
In: Biology
There are a few ways that you can do this, and it’s really quite complicated.
There are things called monocular cues to 3D space. Monocular cues are methods that let you tell the difference between a close object and a far away one with only one eye. For example, occlusion is one example of a monocular cue. Occlusion describes the situation where one object covers the view of another object. The object being obstructed is typically seen as the one being further away.
Another monocular cue is relative size. Simply put, smaller things are typically seen as being further away. Relative size is often paired with a texture gradient, where closer things are typically seen closer to the “bottom” of your visual field, and things farther away are seen towards the “top.” You also have an implicit understanding of how big or small things “usually” are. If you see a coke can and a person, and they look almost the same height, you can infer that the person is probably very far away since you have a decent idea of about how large a coke can is.
You are also born with an implicit understanding of properties of the atmosphere. Things further away have to pass through more air, and so the light reflecting off of them is more scattered and appear fainter and more distance. This is called aerial perspective, and its another clue to which things are further away and which things are closer to you.
You also have the tool of linear perspective. Imagine a road going off into the distance. In this case, the two sides of the road are parallel, but from your perspective they will converge onto a single point called the vanishing point. Linear perspective helps you understand that the vanishing point is probably very far away.
There are also motion cues that you can use to determine depth. When you’re driving in a car and look out the side window, objects close to the car like the railing on a bridge seem to zoom past, but that tree in the distance appears to move a lot slower. This is an example of something called motion parallax, and it’s a situation where you implicitly use outside influences to gain an understanding of depth.
These are all monocular depth cues, and they’re all very basic. They seem obvious to you, but only because they’re so hardwired into your brain– don’t take them for granted, since such a well designed visual system took millions of years to evolve.
There’s also stereopsis. I won’t get very much into this unless someone specifically asks, since it gets pretty complicated pretty quickly, but here is the short version. The nature of the way your eyes are set up is that they see slightly different images. There is an imaginary circle going around your head called the horopter. Again, without going into the geometry, objects that fall on the horopter are seen by the “same” cells on each retina– they fall on the same relative position. As objects get farther from the horopter in either direction, the disparity the two retinas gets larger and larger. This is another way that you can tell the difference between close objects and objects that are farther away. How does your brain take the fact that the two eyes see slightly different images and combine them into a single coherent perception of the world? That’s a fascinating question that I won’t go into here, but I’m also happy to talk about.
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