Why can’t our eyes focus on everything at once?

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Like staring at your own hand and the background is blurry. Why not be able to see everything equally as if we’re looking at it directly?

In: Biology
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You can see everything at once. However, you can’t “focus” as in mentally acknowledge what you’re not looking at.

For example, look at this **DOG**. While staring at that word, can you read the paragraph above? It’s very difficult to do this.

I think that’s what you mean when you say “why can’t our eyes focus on everything at once”. The answer to that question is because your brain will ignore information that it seems unnecessary. It is not necessary to read the paragraph above when you look at the word **DOG**, and so your brain will not process the words above into thought.

Your eye has a lens in it. The lens is subject to the laws of optical physics, and that produces an effect called “depth of field”. Since the light sensitive surface of the eye is almost a sphere, the region of space which can be sharply focused is also a sphere – all the points that are the right distance from the eye. The eye’s lens is adjustable with tiny (very strong) muscles, so you can move the distance in and out. The small aperture of the eye (the black pupil) gives more depth of field, so objects that are about the same distance from the eye as the point of focus are also pretty sharp. Objects far from the point of focus, either nearer to the eye or farther from it, are out of focus (blurry).

Look at camera optics or eye optics to get a graphical representation of what everyone here will talk about. Words alone aren’t great.

Ultimately, we can’t because the eyes/cameras have sensing areas that aren’t single points. They have to have an in-focus image projected onto the sensing area. Due to the properties of optics, that means we can’t focus on nearby stuff and far away stuff at the same time; the light’s geometry just doesn’t allow it. Additionally, for eyes, we have two of them. That allows for depth perception, but further makes it more difficult to have a wide range of focus.