Why is everything not just always in focus why do our eyes have to focus on certain things while making the forground and background unfocused?
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This isn’t a true ELI5 answer, but hopefully it can help.
Extend your arm out in front of you and put your thumb up. If you focus on your thumb, you see a single thumb, but if you focus on something in the far distance beyond, you now see two fuzzy thumbs. If you then cover eye or the other, those 2 fuzzy thumbs will become one clear thumb, but shift left/right relative to the background depending on which eye is covered. You’ll also see that if you focus on something far beyond, the thumb will get fuzzy, but only around the edges (you won’t see two thumbs like you do when both eyes are open).
So what’s happening? Each eye can only see in a straight line, and we have a space between our eyes, so we see a slightly different image with each eye. Move your thumb closet/further away from your face and the difference in images gets worse/better. An image is in focus when these two imaginary lines cross each other (both eyes are looking at the same spot). This is why if you try and focus on something right in front of your nose, you physically have to “cross” your eyes to see it clearly.
Now in the case of covering one eye, it’s the same issue, just at a much smaller scale. Our eye’s image plane (the retina), isn’t a single point, but a small surface. This mean that the incoming light can hit on any point and thus create a slightly different image for our brain to interpret. As someone mentioned previously, the smaller this area becomes the more in focus the entire image will appear because there is less difference between each image until you get to a point where the area is small enough that the angular difference between the edges on the image plane is small enough that we visually can’t see a difference between all of the images created and it looks clear to us.
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