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Let's first see the mechanism of working of concave lens for correction of myopia. If a myopic eye has a near point less than infinity say 1 meter(m) then we use a concave lens of focal length 1, so that the rays from infinity appear to come from 1 meter before reaching our eye lens.

Now, consider a scenario where a person with healthy eyes tries to see a object at infinity using a concave lens . Similarly, the rays from infinity would appear to come from 1 meter and the person will be able to see the object simply because he/she can see an object placed at 1 meter clearly without weraring a spectacle.

The argument shows that the a healthy person can clearly see objects at infinity even after wearing a concave lens. However, in real life I don't think this is true. My friends with healthy eye say that the vision is not clear after wearing glasses. Why is it so?

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  • $\begingroup$ Re, "My friends with healthy eye say that the vision is not clear after wearing glasses." That does not sound like a rigorous experiment. A better experiment would have an objective definition of "clear," and an objective definition of "healthy eye," and it would talk about _at what distance the person with healthy eyes is or is not able to clearly see the target. Also note: Lighting level matters because your pupils contract when looking at a brighter scene. Smaller aperture (pupil) ==> greater depth of field. $\endgroup$ May 7, 2020 at 16:56
  • $\begingroup$ P.S., The objective measure of "clear" that opticians use is the good old eye chart, with rows of letters or other symbols reproduced at a number of different sizes. The smallest row that the experimental subject is able to read is a measure of how "clearly" they were able to see the chart. The same eye chart, without any lenses, probably also could be used in the test for who has "healthy eyes" and who does not. $\endgroup$ May 7, 2020 at 16:57
  • $\begingroup$ So are you telling that there is possibility for a healthy person to see distant objects after wearing spectacles(concave) $\endgroup$
    – anonymous
    May 7, 2020 at 17:19
  • $\begingroup$ it's not the near point it should be the far point. $\endgroup$ Apr 23 at 5:49

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As Solomon Slow pointed out, there are several problems with your formulation. But I think I understand what you asked and I will rephrase parts of it.

1-Your friend has what would be described as 20/20 (6/6 for the metric folks) vision (ability to recognize a 1arcmin feature 6m away, or 20ft), or better

2-The glasses he used were indeed for myopic correction and not other types

Then in theory he should be able to discern infinity as you formulated.

But then there are some biological constraints, and here my lingo may be wrong, but I hope I can explain things concisely.

When focusing to infinity, a persons lens' muscles are relaxed. This mean the eye lens is at its flattest. Typically, the brain also "commands" the eyes to look in a parallel fashion, meaning that if I am looking at a tower 2km away, both eyes have a very small converging angle between them (the angle is basically $2*\arctan(d_{\text{eye to nose}}/2\text{km})$). While looking at things at 1m distance, your converging angle is much wider (you are a bit cross-eyed). Biologically speaking, our brain does this automatically, both the focusing and the convergence, and for some people that do not need glasses, seeing through glasses makes this matter hard. It's NOT NORMAL for the eye to be relaxed while converging to a point nearby, nor is it normal for the lens' muscles to be contracted but converging to infinity. It becomes hard to converge both images on top of each other while having them both sharp.

However, through habituation it should be possible to train the brain to relearn how to look through those glasses.

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  • $\begingroup$ So you are telling that if a person with healthy eye tries to see object at infinity using spectacle, he has to contract his lens but has to converge his eye to infinity which is not possible, right? However my doubt is if he uses a diverging lens the object would appear to come from 1 meter. So why should his eyesight converge to infinity $\endgroup$
    – anonymous
    May 8, 2020 at 15:35
  • $\begingroup$ It is possible to converge to infinity, while straining your eyes to contract the lens, like I said, its possible to train that. You need to understand that the diverging lens creates a virtual image, and that each eye works by itself. Until a real image is formed the virtual image is not "1 m away". Putting it differently, think of converging glasses and a paper behind. You see 2 independent images. Thats the same with the eyes. The converging point is still infinity. Your argument would be slightly more correct if you would look through a giant single lens. $\endgroup$ May 8, 2020 at 18:24
  • $\begingroup$ Putting it in different words: although the curvature of the phase fronts after each lens correspond similarly to that of an object 1m away, the "information" is still parallel. For both real images created in the back of your eye to be overlapping correctly, your eyes need to be looking straight ahead and not to an object 1 m away. $\endgroup$ May 8, 2020 at 18:29
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Depends on the person, glasses, etc. Though if someone can see clearer through someone else's glasses than using their naked eyes alone, it's safe to say that he or she doesn't have full 20\20 or 6\6 eyesight. Either that or there's some underlying problem that's not exactly related to refractive errors (like many people with photophobia [extreme sensitivity to light] find they see clearer with certain tinted glasses than without). Also, it's possible for the eyes to be healthy and still have some kind of refractive error, because "healthy" in this context means free of the various eye diseases.

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Your friend might have hypermetropia.Maybe his near point is situated at a distance more than 1 meter,which means he can not see things that are within 1 meter range from his eyes,so he cannot see at infinite distance because the light rays are seemingly diverging from 1 meter distance because of a concave lens.

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