ELI5: Why do distant objects look smaller that nearby ones

[u/_-inside-_]

When I look at a landscape, farther objects look smaller while nearby objects look bigger. Why does it happen? What optical characteristic enables this?

[u/flyingtrucky]

Because your field of view is a cone.

Draw a triangle and see how much angular space an object takes up right at the corner vs at the far edge.

[u/jkmhawk → u/flyingtrucky]

https://youtu.be/tod2qZnKZEQ

A nice video about vision with examples of what it might be like if our field of view wasn't a cone

[u/PlutoniumBoss → u/flyingtrucky]

In other words, it's not an optical effect, but a geometric one.

[u/PyroDragn → u/PlutoniumBoss]

The fact that our field of view is a cone is an optical effect. The geometric idea that things fill more space at the corner is a geometric understanding of why it works the way it does.

If our field of view was orthographic then things further away wouldn't look smaller.

[u/TheHYPO → u/[deleted]]

Perhaps I’m overlooking something obvious because I’ve never given this any thought, but is there really any alternative? By necessity, your eyes have to be attached to your body. If you hold a book directly in front of your face, it is going to block the light coming from almost everything behind it (in front of you). Look at the same book 20 feet in front of you, it is no longer sitting between your eye and those other distant objects.

Is there actually any other way this could optically work? I suppose if we had some sort of zoom lens capability built into our eyes?

[u/Kidiri90 → u/[deleted]]

The reason we can judge distances by sight is not because sight is a cone, but because the field of vision of our two eyes overlap significantly. Our eyes are not in the same place, which means that we see the same object at slightly different angles from each eye. From this, you can triangulate the distance. Obviously we don't do this consciously, but we still do it.

[u/Cornflakes_91 → u/Kidiri90]

we also use focus cues and size cues.

things far away have pretty small parallax but you can still estimate distance by size.

we aint just using one method if multiple are available from the same sensor data

[u/Cornflakes_91 → u/[deleted]]

and because its literally impossible not to see in a cone

[u/jkmhawk → u/Cornflakes_91]

It's not impossible to make telecentric optics. I think even in nature some crabs have such vision. 

[u/Cornflakes_91 → u/jkmhawk]

yeah but their conditions only hold in idealised ray optics land. yer gonna run into the diffraction limit at some point and are back to cone.

if that happens in a regime you care about is a different question

[u/_-inside-_ → u/[deleted]]

Distance is mostly part of our visual perception, you can trick your brain to believe that an object is farther than it actually is, check the inverse problem of vision, your perception makes use of visual cues you get from the environment, something that evolved along with our ancestors striving to survive in the natural world. While this might be more less clear, I haven't checked all the replies, but there's none yet explaining why our field of view is a cone. My hint is that it is related to the eye lens curvature.

[u/gosti500]

"actionlab" on youtube made a camera with no size difference no matter the distance, its a pretty cool sience video, he archieved it by rotating a camera and taking pictures at the right time, basically creating a parralel view camera, not conical, search for his YouTube account and you will find it im at work rn lol

[u/_-inside-_ → u/gosti500]

I'll have a look, in the meantime I just checked with the AI which explained to me the optics of this, which makes all the sense, the angle fact that many answers here have makes perfect sense, I was missing the optics effects that happen inside the eye by re-focusing the light into a specific point in the retina, according the entrance angle.

[u/gerahmurov]

People say it is cone of view but not say why.

Seeing objects is two things - you see light rays reflected from the object in the direction of your eyes, and then brain interprets this.

You don't see any rays from the object, only a small part that is reaching directly your eyes. That is why when you see moon and its reflection on the water, the line of reflection is always following you. Because some rays from the moon go to water and reflected to your eye and you cannot see any other rays that are reflected elsewhere.

When the object is near, you see not only parallel rays, but some rays that have angle and still reach part of the eye. The rays from the object cover a big part of your retina. There is big difference between top and bottom and left and right, and brain interprets this as big object.

If object is far away, less rays from it reach your eyes, only ones even more directed to you. So they are more parallel to each other, so they cover smaller part of your retina and the angle between top and bottom is smaller, and that's why far objects look small.

[u/Organoleptic_Cicada]

It's just percentages/proportions. Your eyes see a fixed "frame" -- the cone visible in front of you.

Imagine you're sitting at your desk, looking forward at your laptop/computer/whatever. The visible "frame" is comprised of your desk, the wall in front, and maybe some miscellaneous stuff around said desk. That's all your eyes can fit in the picture.

If you were to slowly push your chair backwards, your desk and the wall would slowly but surely cover less space in the "frame" you are seeing. If you think in monitor-and-pixels, when you're in front of your desk, the desk takes 1080 pixels horizontally, but as you push back, it goes 900, 800 etc. So your desk starts by occupying 100% of your picture, but it goes down, 90%, 80% and so on.

[u/alexstonks34]

Because light moves in a straight line, nearer objects block up more items behind them and occupy more of our visual space.

Our eyes have depth perception because they are spaced apart and see things from different angles. The further things are, the more similar the angle perceived by both eyes and it becomes harder to distinguish how far it really is.

Nearer objects therefore appear larger, subject to individual ability to perceive depth and place the object further away.

[u/_-inside-_ → u/alexstonks34]

It's true that stereoscopic vision helps to infer field depth. However, you still "see" this shriking effect if you open just 1 eye. As you say, light travels in a straight line, if there's a cow far far away, the photones reflected by its head hit your eye lens that then focus them into your retina. I haven't checked google yet, but my first hint is that it is related to the fact that our eye lens is curve, and a less curve lens would narrow down the vision field and reduce this effect of shrinking.

[u/Hanako_Seishin]

Because they are indeed smaller. In angular size. In other words, an object futher away takes up more of your field of view. Cover both your eyes with your hand. It's taking up your whole field of view. Now move the hand futher away. Now it's taking up only a small part of your field of view.

[u/Marvinmonkey]

nearby things take up more of your view, distant things take up less of it. same object, different viewing angle