Why is it that I am not able to read a book without moving my eyes if the entirety of the page is within my field of view? Why do I have to center my eyes on an object to observe it fully? And why is it that I am still able to view changes in surroundings in the edges of my field of vision despite there being supposedly no way to focus on them from that angle?
Is it due to our brain’s capacity to absorb a finite amount of visual information at a given moment or is it a physical flaw in the structure of our eyes?
I’m sure there’s a physical answer as to why a spherical lens can’t focus the entire image (or maybe it can, physics is not my area and I think it shows haha).
But the biological explanation is that your retina isn’t uniform, instead you have a very small region called fovea where the vast majority of your cone cells are concentrated. If you could take an instantaneous snapshot of your vision, like a picture, you’d be scared to see that everything is grossly blurry and distorted save for a very small circle where the image is very clear - that’s the fovea. Your brain takes multiple images where different parts of a scene are focused on the fovea to create a composite end result that looks better. The rest of your eye still captures light, but with less detail, and therefore it’s mostly dedicated to getting broad positioning of objects, noticing fast changes in movement, tracking peripheral motion, and so on - not on focusing on text or small details.
Curiously, you also have a circle that would be completely black if your brain didn’t fill it in - it’s the blind spot left by the insertion of the optical nerve, where your retina can’t capture anything.
This is what interests and distresses me about the mysteries of the human body. I once saw a video about capturing a person’s recollection of his memory of a video clip by scanning his brain activity. The result was really obscured and blurry, but it actually did resemble the clip, which was deeply disturbing. I would have never known my actual vision is vastly different from what my brain makes me perceive to be.
I think a useful way to think about it is that your perceptual brain isn’t in the business of making you see, or hear, or anything like that - it’s in the business of giving you an accurate-enough-to-be-useful idea of what’s around you. You see the world as being sharp and stable and consistent even though the literal visual signals going from your eyes to your brain aren’t… because the world is sharp and stable and consistent. Or at least it’s enough those things that it was useful for the brain to evolve to generate that specific perceptual experience. The signals coming from your eye are just (some of) the information the brain uses to generate that experience.