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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It’s a fact of light that the bigger the opening into which light comes in, the worse things can be in and out of focus. If you had a literal pin-hole camera with the “lens” just being an exceptionally tiny hole through which light comes into the film exposure area, everything would pretty much always be in focus. However the amount of light coming in would be very small and so you’d need to be very sensitive to light, or a camera might need a very long exposure time for its film.
Bigger openings let more light in, making designing the light sensor or film easier since it doesn’t need to be as sensitive. But focusing the light properly now becomes a problem.
TV and movie cameras are huge, great for capturing detail, but oh boy do they need to be focused properly….
For our brains there are two reasons: attention and 3-dimensions
If everything is in focus there are too many things to pay attention to. Your brain isn’t capable of recording everything it sees, so focusing visually helps it focus attention.
Second is that we see in three dimensions. This has two parts.
– The way lenses work, they ‘magnify’ things at a certain distance. That action to make one distance clearer makes another distance less magnified, or out of focus.
– if everything is equally in focus it’s harder for the brain to tell which is in front and which is in back. If those things are still, it’s nearly impossible. Having two eyes helps the most, but focus/non focus helps also.
As a sidebar – hold your two thumbs together out at arms length. The area of your thumbnails is what is in focus at any one time. This is called the fovea. We “see” everything in focus because the eyes continually move and your brain stitches it all together. You brain stitches what your eyes hve seen.
If you have ever been doing something and put, say a small nail, down next to you without looking and then after a few minutes look for it. It may have “disappeared” because your brain has used its last image of that area.
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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