I was holding my 8 month old son today on balcony and started to wonder as his pupils changed size to focus on further objects, why light bouncing off surfaces near and far differ so, that the eye understands that difference and has to adjust to “see” and translate that light to brain. What causes the image to be blurry, if the pupil remains larger while we look at objects that are far?
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Adjusting pupil size is to correct for changes in brightness, not distance.
Focus is needed because the lens of your eye has a size, up to 7 mm across fully dilated. The world looks slightly different from one side of your lens than from the other, because of the different points of view or perspective. The lens can correct for this effect, but only for objects at certain distances. It’s fundamental geometry. It’s like the way your two eyes have to converge to look at nearby objects but then both eyes look in the same direction to look at distant objects.
In bright light, your pupils contract, making the lens of your eye effectively very small. This reduces the differences in points of view across the lens and focusing becomes much less critical and, outside on a sunny day, things both near and far can be in focus at once. In a darkened room, your pupils dilate and only objects at a very limited range of distances are in focus.
FYI, pupil vs cornea vs lens are all different concepts. Focusing is about lens shape. Pupil is about light exposure.
Imagine a cup of coffee that is 1 foot from your eye. Lets say you pick an arbitrary point in the middle of the cup to ‘focus on’.
What you want, is for all the light that bounces off that point and reaches your eye, to hit the back of your eye as a single point as well (imagine the back of your eye as the tiny ‘screen’ that some TV watcher in your brain is viewing). If it hits multiple points, it will look like multiple copies of the image at different overlapping points on the screen. This is basically what ‘blurry’ objects are.
The light from that point is spreading out as it approaches your eye. Imagine every ‘point’ as a tiny sun, light always spreads out in every direction. Your eye lens is pushed/pulled by muscles into a curve that mathematically refocuses all the light entering the eye, as perfectly as possible, back into a point. How does your brain know? Well if it is wrong, you get a ‘blurry’ image because some of the light will hit higher and/or lower, making weaker versions of the point exists above/below/to the right/left of the [object]. (https://pixelcraftphoto.files.wordpress.com/2019/12/circleofconfusion.jpg). This creates blurriness and your brain trains, every day, all day, for years, to learn to unconsciously bring things into focus. This is a huge part of your early years brain development.
What these diagrams often fail to show is just how precise that focus point is, outside of which the lens curve isn’t perfect, and how much processing of the image your brain is doing to clean up blurriness around the focus point.
Going back to the coffee cup, when the coffee is ‘in focus’, only a point/small area on the coffee cup is truly in focus. Even other parts of the coffee cup will be far enough away to be blurry if you draw all the light lines, and if you try to focus your attention on them without shifting your focus, you can see this blurriness although your brain hates trying to do that.
That is why we have to move our eyes to scan words on a page, even though all of them are at the same distance from the eye. Only that specific focus point is perfectly clear.
Now put the coffee cup 4 feet further back. The light from the same point on the coffee cup has longer to get to our eye, which means the maximum angle between different rays is actually smaller (think of an object at infinite distance, all ray lines that reach our eyes would be parallel). So we need a different lens shape and size that doesn’t redirect as much for it to be in focus. We have to adjust it to be flatter.
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