Nearsightedness/ Farsightedness confusion

[airjavaid]

I understand the basic principles behind both myopia and hyperopia, but I am a little confused as to why they are true.

So in myopia the distance from the lens to the retina is longer than normal, requiring the person to use a diverging lens to move the focal point back to the retina so that they can focus on distant objects. Now the question is, how is someone who is suffering from nearsightedness able to focus on near objects in the first place? If its true that the lens to retina distance is larger than normal, then how would they be able to focus near objects on their retina, especially considering the fact that their lens produces further convergence of the focal point when they looking at an object near to them?

The same goes for hyperopia: if the distance between then lens and retina is closer than normal, then how are people who suffer from farsightedness able to focus on distant objects?

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[neurodoc]

I understand the basic principles behind both myopia and hyperopia, but I am a little confused as to why they are true.

So in myopia the distance from the lens to the retina is longer than normal, requiring the person to use a diverging lens to move the focal point back to the retina so that they can focus on distant objects. Now the question is, how is someone who is suffering from nearsightedness able to focus on near objects in the first place? If its true that the lens to retina distance is larger than normal, then how would they be able to focus near objects on their retina, especially considering the fact that their lens produces further convergence of the focal point when they looking at an object near to them?

The same goes for hyperopia: if the distance between then lens and retina is closer than normal, then how are people who suffer from farsightedness able to focus on distant objects?

The lens is rot rigid (like a glass lens). It is elastic and is connected to small muscles that tug on it to change its shape. This allows for accomodation. With age, the lens becomes less elastic. Young mypopes have longer eyeballs and can see well up close, but as they age they have problems focusing up close. I'm sure someone from Ophtho will chime in soon. The point is that this accomodative change in the shape of the lens is what explains your questions.

 

[airjavaid]

I understand what your saying about accommodation, but it seems like the accommodation would worsen the situation. For example, if a myopic person, whose focal point is in front of the retina in the first place, tries to focus on something close up, which they supposedly should be able to do if they aren't suffering from presbyopia as well, then their lens will become more "thick" in the middle, therefore, making the focal length even shorter. Now why is it that despite their decreased focal length, which is now even further away from the retina, they are able to focus on close up objects? See what I'm getting at? It seems like they should not be able to focus on close up objects, especially because their lens undergoes accommodation when looking at close up objects.

 

[neurodoc]

I understand what your saying about accommodation, but it seems like the accommodation would worsen the situation. For example, if a myopic person, whose focal point is in front of the retina in the first place, tries to focus on something close up, which they supposedly should be able to do if they aren't suffering from presbyopia as well, then their lens will become more "thick" in the middle, therefore, making the focal length even shorter. Now why is it that despite their decreased focal length, which is now even further away from the retina, they are able to focus on close up objects? See what I'm getting at? It seems like they should not be able to focus on close up objects, especially because their lens undergoes accommodation when looking at close up objects.

The ciliary muscle (CM) is responsible for "accomodation." When it relaxes, the lens asumes a more spherical shape ('plus" lens power). With age, the lens becomes more inelastic, so with age myopes are less able to focus "up close." So are hypermetropes...., but hypermetropes are more impaired with this close-focusing to begin with. It seems pretty clear cut to me, but I'm just a neurologist. I'd like to hear from an opthalmologist to confirm my opinion.

 

[indianjatt]

I understand what your saying about accommodation, but it seems like the accommodation would worsen the situation. For example, if a myopic person, whose focal point is in front of the retina in the first place, tries to focus on something close up, which they supposedly should be able to do if they aren't suffering from presbyopia as well, then their lens will become more "thick" in the middle, therefore, making the focal length even shorter. Now why is it that despite their decreased focal length, which is now even further away from the retina, they are able to focus on close up objects? See what I'm getting at? It seems like they should not be able to focus on close up objects, especially because their lens undergoes accommodation when looking at close up objects.

Can't you look at the thin lens equation? If the object is close to the lens, which is what happens when the object is very close to your eye, the focal length has to decrease, which is exactly what you described. So in order to focus on a close by object, the focal length must decrease since we can somewhat assume that the image distance of the eye is constant.