eli5 why distance objects appear smaller, and why not only blurrier while size is the same for the naked eye.

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Think about how many cars fit in a circle. If the circle is 20 feet across, you might be able to get 4-6 cars. If you make the circle 200 feet across you get about 50 cars.

If you stand in the middle of the circles you can’t see all the cars, of course, but you can see 1/3 to 1/2 of them. To see more of them, they have to get smaller (in angular width).

Light doesn’t just come into your eyes straight on, it comes in at an angle. The field of vision for a single eye is around 135 degrees horizontally and 180 degrees vertically.

The further away from your eye, the more horizontal and vertical distance is covered by that angle. That is, the farther away you’re looking, the more you can see.

So while a further away object is the same absolute size it is smaller relative to your entire field of vision.

Grab a sheet of paper. Draw a line. Thats your retina. Now drawn a point somewhere in front of it. Thats the hole the light enters your eye. Now draw an object in different distances like 2 points connected with a line. Then draw a line from the points of the object through the point in front of your retina and mark where the line intersects the retina. Drawing the object at different distances will make it appear smaller or larger on the retina. Go see for yourself. In a nutshell this is how our eyes work.

So you’re looking at a big car, let’s say a pickup truck. When standing right in front of it (touching it with your nose), can you see all of it at once? Probably not, because it fills your entire field of vision. That means, the light from the far left side of the truck is coming into your eye at an extreme angle, which is radically different than the angle from which the light from the far right side of the truck is entering your eye. Since your eyes have a limited “field of vision”, you are likely not able to actually view both sides at the same time – at point blank range, that is.

Now you step back… about a mile or a couple kilometers or whatever long range unit you’re using. Now you look at that same truck again. Can you now see both sides? What’s the difference in the angles of the light from the two sides? Basically no difference, compared to your entire field of vision. Wouldn’t it be really weird if you’d see a ginormous blurry truck blocking your view from miles away?

Because that’s how perspective works. Your eyes aren’t just enlarging a picture. They’re seeing an actual object that’s far away.