This might be an unremarkable observation but the further away someone is to an object the smaller it becomes and vice-versa. From an evolutionary standpoint it doesn't seem to be any type of benefit to perceive further objects as smaller object. On the other hand the alternative means that objects do indeed get smaller, with distance, relative to one another, which seems a bit silly.

What is the best model to understand this phenomenon?

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    It's kind of a flabbergasting question... (^_^;) It's a very basic problem of geometry, the light going from objects that are further to your eye form a smaller angle and hit a smaller part of your retina, so they look smaller even if their size do not change. You can check it easily by drawing on a paper. From an evolutionary viewpoint, perspective helps us evaluate distance (along with binocular vision), and it would on the contrary make no sense that our vision corrects it. It would also require a pretty convoluted signal processing. – armand Jan 6 at 10:59

In Euclidean space, the surface area of a sphere of radius r grows as r2. The larger the physical area, the more objects of a given physical size can fit in it, and so the smaller they must be in angular size to fit in your field of view.

In a space of uniform positive curvature (like Einstein's static universe), the surface area of a sphere is sin2 r/R. As a result, past a certain point (r = ½πR, the "equator" of the universe if you imagine yourself to be at a pole), more distant objects appear larger than closer objects. An object at the antipodal point of the universe will fill your entire field of view.

In real cosmology, spatial slices are flat, but the speed of light is finite and the universe is expanding, and so a similar thing happens. With current best-fit parameters, sizes start increasing past a redshift of z ≈ 1.6, or a lookback time of around 10 billion light years. GN-z11, with a redshift of 11.09, looks as large as if it were less than 3 billion light years away, with a redshift of 0.25. If there were another galaxy of the same size and intermediate redshift along the same line of sight, it would appear as a smaller galaxy only partly obscuring GN-z11.

So the law of perspective is definitely a property of reality, and tells you something about the shape of the universe.


Well I don't know much about evolution but this can be explained easily with help of Physics.

To understand the answer you need to understand the underlying concept of reflection of light from an object which helps you see the objects when it enter your eyes. When a light hits an object it is scattered in all direction.

So if you stand close to an object you can only see a part of that object because the light scattered by other parts is not entering your eyes and you need to either move your head or eyes to see the rest of the object. This makes you perceive that object as big.

Whereas when you are standing far away from an object more light is entering in your eyes which was scattered by more area of the object, hence you can see a lot more of it compared to when you were close to it, that makes you perceive that object as small.

But in reality the object is not changing the size itself, it's the amount of information you are getting that makes your brain perceive objects small and large, just like an illusion.

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    "when you are standing far away from an object more light is entering in your eyes"? It seems like you're talking about something akin to the dolly zoom effect. I'm not sure that's much of a distance cue at all – do these people look like they're getting closer? It's certainly not as much of a clue as the actual change in angular size of objects as a function of distance, which manifests in part as a decrease in the amount of light entering your eyes at larger distances. – benrg Jan 6 at 23:10
  • I was not talking about Dolly Zoom effect, I didn't know the name of the concept, glad I was able to learn something new. However the amount of information we get in changing as we move, it's mentioned in the video ( 5:40 ) you sent. Did you watch the entire video? Dolly zoom an in-camera effect that appears to undermine normal visual perception. The effect is achieved by zooming a zoom lens to adjust the angle of view while the camera dollies toward or away from the subject in such a way as to keep the subject the same size in the frame throughout. – White Mars Jan 7 at 7:25

This article, Sizing Thinks Up, describes how perceptions rely on a thoughtlike process which German physicist Hermann von Helmholtz called unconscious inference:


We have often emphasized in our column that even the simplest act of perception involves active interpretation, or “intelligent” guesswork, by the brain about events in the world; it involves more than merely reading out the sensory inputs sent from receptors. In fact, perception often seems to mimic aspects of inductive thought processes. To emphasize perception’s thoughtlike nature, von Helmholtz used the phrase “unconscious inference.” Sensory input (for example, an image on the retina at the back of the eye) is interpreted based on its context and on the observer’s experience with, and knowledge of, the world. He used the word “unconscious” because, unlike for many aspects of thinking, no conscious cogitation is typically required for perception. By and large it is on autopilot.

We can explain the illusion in terms of a visual effect called size constancy; if two objects of identical physical size are at different distances from a viewer, they are correctly perceived as being the same physical size, even though the images cast by them on the retina are different sizes. Quite simply, the brain “understands” there is a trade-off between retinal image size and distance and, in effect, says, “That object’s image is small because it is far; its actual size must be much bigger.” To evaluate distance, the visual system uses various sources of information called “cues,” such as perspective, motion parallax, texture gradients and stereopsis. It then applies the appropriate correction for distance in order to judge true size.

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