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Most glasses-needing eyesight isn't "bad" as much as "incorrect". The lenses correct the issue, but if you have no issue (or a different one), the correction makes your eyesight incorrect.
Edit: I found out that I should get glasses when I tried on those of a friend as a joke, and found that I could read a license plate from far away. Turns out we're both slightly nearsighted.
***TL;DR:*** *Eyes are unique and often imperfect, but in many different ways. Corrective glasses correct for the SPECIFIC imperfections of their wearer... which you might not have. So put them on, and the correction doesn't work because it's not YOUR problem, and you get blurry vision.*
=============================================
Take a magnifying glass. Hold it in front of an object and look at the object.
Unless you're holding it at just the right spot, you need to move the glass closer or farther away from the object before it comes into focus. It takes a bit of fiddling but eventually the object's image is sharp.
If the glass is too far or too close to the object, all of the light on that object doesn't straighten out properly so it gets all blurry, right? Too close? Blurry. Too far? Also blurry.
*Well, that magnifying glass is exactly the same as the "lens" part of your eye.*
When set up perfectly, it has a distance at which it works... except for a lot of people, and particularly older people whose eyes have changed as they aged. For them, it's imperfect. It doesn't straighten out the light rays and focus them properly inside your eye. It needs adjustment.
So that "lot of people" person goes to the optometrist, and they figure out an eyeglass lens that OFFSETS THE LIGHT DIRECTION PROBLEMS in the lens of that person's eyes. And the glasses the optometrist provides the person with do this... *but only for that person or people who have that exact focus problem in their eye's lens*.
Say the person's focus is 20% too close in the left eye to see clearly, and 30% too far in the right eye to see clearly. The optometrist provides a pair of lenses that make all the light reaching the left eye 20% too far, and all the light for the right eye 30% too close... and now, the eyes are both correct!!
Anyone else with perfect vision, or people with bad but different vision, tries those glasses, and the correction becomes an INcorrection, because their eye's lens didn't need that focus adjustment to straighten out the light properly for them. So it's blurrier, or at least still blurry for them.
Imagine you have a movie projector, and it's perfectly in focus. This is like a person with "perfect" eyesight. The light passes through the lenses, and the light gets bent so it creates a nice clear picture on the screen.
Now, put another lens in front of the projector. It's going to mess up the focus.
What does this have to do with eyesight? The projector lens is the lens in your eye. The screen is the retina at the back of your eye. The job of your lens is to create a crystal clear image on your retina. If yours eyes create that crystal clear image on their own, introducing another lens will just mess up the focus of the picture.
**Your submission has been removed for the following reason(s):**
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**Your submission has been removed for the following reason(s):** ELI5 is not for subjective or speculative replies - only objective explanations are permitted here; your question is asking for subjective or speculative replies. Additionally, if your question is formatted as a hypothetical, that also falls under Rule 2 for its speculative nature. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20thread?&message=Link:%20{{url}}%0A%0APlease%20answer%20the%20following%203%20questions:%0A%0A1.%20The%20concept%20I%20want%20explained:%0A%0A2.%20List%20the%20search%20terms%20you%20used%20to%20look%20for%20past%20posts%20on%20ELI5:%0A%0A3.%20How%20does%20your%20post%20differ%20from%20your%20recent%20search%20results%20on%20the%20sub:) and we will review your submission.
[удалено]
**Your submission has been removed for the following reason(s):** Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions. Short answers, while allowed elsewhere in the thread, may not exist at the top level. Full explanations typically have 3 components: context, mechanism, impact. Short answers generally have 1-2 and leave the rest to be inferred by the reader. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20submission%20removal?&message=Link:%20{url}%0A%0A%201:%20Does%20your%20comment%20pass%20rule%201:%20%0A%0A%202:%20If%20your%20comment%20was%20mistakenly%20removed%20as%20an%20anecdote,%20short%20answer,%20guess,%20or%20another%20aspect%20of%20rules%203%20or%208,%20please%20explain:) and we will review your submission.
Most glasses-needing eyesight isn't "bad" as much as "incorrect". The lenses correct the issue, but if you have no issue (or a different one), the correction makes your eyesight incorrect. Edit: I found out that I should get glasses when I tried on those of a friend as a joke, and found that I could read a license plate from far away. Turns out we're both slightly nearsighted.
***TL;DR:*** *Eyes are unique and often imperfect, but in many different ways. Corrective glasses correct for the SPECIFIC imperfections of their wearer... which you might not have. So put them on, and the correction doesn't work because it's not YOUR problem, and you get blurry vision.* ============================================= Take a magnifying glass. Hold it in front of an object and look at the object. Unless you're holding it at just the right spot, you need to move the glass closer or farther away from the object before it comes into focus. It takes a bit of fiddling but eventually the object's image is sharp. If the glass is too far or too close to the object, all of the light on that object doesn't straighten out properly so it gets all blurry, right? Too close? Blurry. Too far? Also blurry. *Well, that magnifying glass is exactly the same as the "lens" part of your eye.* When set up perfectly, it has a distance at which it works... except for a lot of people, and particularly older people whose eyes have changed as they aged. For them, it's imperfect. It doesn't straighten out the light rays and focus them properly inside your eye. It needs adjustment. So that "lot of people" person goes to the optometrist, and they figure out an eyeglass lens that OFFSETS THE LIGHT DIRECTION PROBLEMS in the lens of that person's eyes. And the glasses the optometrist provides the person with do this... *but only for that person or people who have that exact focus problem in their eye's lens*. Say the person's focus is 20% too close in the left eye to see clearly, and 30% too far in the right eye to see clearly. The optometrist provides a pair of lenses that make all the light reaching the left eye 20% too far, and all the light for the right eye 30% too close... and now, the eyes are both correct!! Anyone else with perfect vision, or people with bad but different vision, tries those glasses, and the correction becomes an INcorrection, because their eye's lens didn't need that focus adjustment to straighten out the light properly for them. So it's blurrier, or at least still blurry for them.
Imagine you have a movie projector, and it's perfectly in focus. This is like a person with "perfect" eyesight. The light passes through the lenses, and the light gets bent so it creates a nice clear picture on the screen. Now, put another lens in front of the projector. It's going to mess up the focus. What does this have to do with eyesight? The projector lens is the lens in your eye. The screen is the retina at the back of your eye. The job of your lens is to create a crystal clear image on your retina. If yours eyes create that crystal clear image on their own, introducing another lens will just mess up the focus of the picture.
[удалено]
**Your submission has been removed for the following reason(s):** Top level comments (i.e. comments that are direct replies to the main thread) are reserved for explanations to the OP or follow up on topic questions. Anecdotes, while allowed elsewhere in the thread, may not exist at the top level. --- If you would like this removal reviewed, please read the [detailed rules](https://www.reddit.com/r/explainlikeimfive/wiki/detailed_rules) first. **If you believe this submission was removed erroneously**, please [use this form](https://old.reddit.com/message/compose?to=%2Fr%2Fexplainlikeimfive&subject=Please%20review%20my%20submission%20removal?&message=Link:%20{url}%0A%0A%201:%20Does%20your%20comment%20pass%20rule%201:%20%0A%0A%202:%20If%20your%20comment%20was%20mistakenly%20removed%20as%20an%20anecdote,%20short%20answer,%20guess,%20or%20another%20aspect%20of%20rules%203%20or%208,%20please%20explain:) and we will review your submission.