system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do you calculate how much an object gets smaller, the further is is away?

Is there a certain ratio for this? Like how long is a 1 meter ruler, 1 meter  behind a 1 meter ruler ?
gpt
it has to do with a thing called "arc distance"

you don't see heights and lengths, you see how much of your cone of vision an object takes up. a 1 meter long object 1 meter away from you will take up ~53 degrees of your vision. another meter stick 1 meter beyond that would take up ~28 degrees of your vision. 

the math is the geometry of an isosceles triangle, where the non-equal leg is the object being viewed.