Short sight question 
Author Message
 Short sight question


writes:


Quote:
 CM>Samuel Muttiah) writes:

 CM>
 CM>>       Why do people having short sight see things that are
 CM>>close magnified without their glasses ?
 CM>
 CM>We don't. Like everybody else, we see things far away as smaller
 CM>(smaller subtended angle) and things that are closer as bigger. But
 CM>because we can see things closer than the normal sighted, we gain the
 CM>benefit of seeing them even bigger.

Not quite. The optics of myopia is such that objects held a specific distance
from the unaided nearsighted will appear larger than the same object held
before someone who is farsighted. When glasses are worn - the reverse is true.

 CM>Confusion is caused by the fact that our glasses, if held away from the
 CM>eyes, make things look smaller.

This is only true of concave lenses or the type worn by the nearsighted. It is
not true of convex lenses. As a given lens is moved farther from the eye, its
relative plus power increases resulting in magnification with plus lenses. The
minification caused by minus lenses is only noted in these cases with extremely
highly concave lenses.

Leo Bores, M.D.

--  
Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!14!Leo.Bores



Tue, 15 Sep 1992 03:27:09 GMT
 Short sight question

Quote:


>writes:

> CM>Samuel Muttiah) writes:
> CM>>       Why do people having short sight see things that are
> CM>>close magnified without their glasses ?
> CM>We don't. Like everybody else, we see things far away as smaller
> CM>(smaller subtended angle) and things that are closer as bigger. But
> CM>because we can see things closer than the normal sighted, we gain the
> CM>benefit of seeing them even bigger.
>Not quite. The optics of myopia is such that objects held a specific distance
>from the unaided nearsighted will appear larger than the same object held
>before someone who is farsighted. When glasses are worn - the reverse is true.

I thought I understood optics, but this puzzles me. Let me ask a
specific question: if an object is held at the same distance from a
near-sighted and a normally-sighted person, and _correctly_focussed_ on
the retina in both cases, do you claim that it appears bigger to the
near-sighted? If so, can you explain this?

Quote:
> CM>Confusion is caused by the fact that our glasses, if held away from the
> CM>eyes, make things look smaller.
>This is only true of concave lenses or the type worn by the nearsighted. It is
>not true of convex lenses. As a given lens is moved farther from the eye, its
>relative plus power increases resulting in magnification with plus lenses. The
>minification caused by minus lenses is only noted in these cases with extremely
>highly concave lenses.

If you take a side-frontal view of a specs-wearer, and look thru the
specs to the background beyond, or the outline of the side of the face,
the distinction between minifying or magnifying lenses can _always_ be
made with ease, thus distinguishing the short- from the far-sighted.
But aren't there difficult-to-tell intermediate cases of only slight
aberration? No -- they don't need to wear specs!

--

Department of Artificial Intelligence, Edinburgh University
5 Forrest Hill, Edinburgh, EH1 2QL, UK



Wed, 16 Sep 1992 04:02:38 GMT
 Short sight question

Quote:
Muttiah) writes:

 RS>        Why do people having short sight see things that are
 RS>close magnified without their glasses ?

The nearsighted person has a "near point" that is close to the eyes. All images
with nearpoints that close are enlarged. If glasses are worn the image becomes
smaller.

Leo Bores, M.D.

--  
Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!14!Leo.Bores



Fri, 18 Sep 1992 00:34:47 GMT
 Short sight question

writes:


Quote:
 CM>Samuel Muttiah) writes:

 CM>
 CM>>       Why do people having short sight see things that are
 CM>>close magnified without their glasses ?
 CM>
 CM>We don't. Like everybody else, we see things far away as smaller
 CM>(smaller subtended angle) and things that are closer as bigger. But
 CM>because we can see things closer than the normal sighted, we gain the
 CM>benefit of seeing them even bigger.

Not quite. The optics of myopia is such that objects held a specific distance
from the unaided nearsighted will appear larger than the same object held
before someone who is farsighted. When glasses are worn - the reverse is true.

 CM>Confusion is caused by the fact that our glasses, if held away from the
 CM>eyes, make things look smaller.

This is only true of concave lenses or the type worn by the nearsighted. It is
not true of convex lenses. As a given lens is moved farther from the eye, its
relative plus power increases resulting in magnification with plus lenses. The
minification caused by minus lenses is only noted in these cases with extremely
highly concave lenses.

Leo Bores, M.D.

--  
Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!14!Leo.Bores



Fri, 18 Sep 1992 00:36:47 GMT
 Short sight question

Quote:

> I thought I understood optics, but this puzzles me. Let me ask a
> specific question: if an object is held at the same distance from a
> near-sighted and a normally-sighted person, and _correctly_focussed_ on
> the retina in both cases, do you claim that it appears bigger to the
> near-sighted? If so, can you explain this?

Myopic eyeballs are longer.

BTW, here's an anecdote.  I recently got glasses after having worn contact
lenses exclusively for several years.  When I put them on, I was surprised
at how tiny everything appeared.  A beer can about 3 or 4 meters away
looked like one of the tiny V-8 cans.  This was much more than the size
difference that I remembered from back before the contacts, so I wondered
why.  I tried some experiments and found that, in the left eye, the
perceived angular width of something at that distance did not differ much
with and without glasses (inasmuch as I could see the edges of the object,
of course.  However, I have astigmatism in my right eye.  The contact
correction for this was always excellent, but the glasses correction
always changed the aspect ratio.  On the right eye, this had the effect of
making everything seem wider.  It also had the effect of making everything
within ten meters or so appear much closer than it really was.  My
hypothesis is that my eyes saw roughly the correct angular width but that
my visual system combined it with the incorrect distance information to
result in the impression that the object was much smaller than it was.  
The impression of tininess went away when I looked at an object with only
one eye, which supports this hypothesis.

Since then I have become accustomed to the glasses and somehow changed the
mapping to make the distance information come out correct.  I also seem to
have two separate mappings in my head, one for contacts and one for
glasses, and it is becoming easier to switch between them.  Handy things,
those brains.



Florida State University                      SPAN:     scri::pepke

Disclaimer: My employers seldom even LISTEN to my opinions.
Meta-disclaimer: Any society that needs disclaimers has too many lawyers.



Sat, 19 Sep 1992 03:10:51 GMT
 Short sight question

writes:


 CM>
 CM>>Not quite. The optics of myopia is such that objects held a specific
 CM>distance
 CM>>from the unaided nearsighted will appear larger than the same object
 CM>held
 CM>>before someone who is farsighted. When glasses are worn - the reverse
 CM>is true.
 CM>
 CM>I thought I understood optics, but this puzzles me. Let me ask a
 CM>specific question: if an object is held at the same distance from a
 CM>near-sighted and a normally-sighted person, and _correctly_focussed_ on
 CM>the retina in both cases, do you claim that it appears bigger to the
 CM>near-sighted? If so, can you explain this?

It is explained by the fact that the retina in a nearsighted person is farther
from the nodal point than is the retina of an emmetrope (normal sighted)
person. excluding the blur circles - the image perceived is larger. ther
reverse is true of the far-sighted (hyperopic) person.

 CM>If you take a side-frontal view of a specs-wearer, and look thru the
 CM>specs to the background beyond, or the outline of the side of the face,
 CM>the distinction between minifying or magnifying lenses can _always_ be
 CM>made with ease, thus distinguishing the short- from the far-sighted.
 CM>But aren't there difficult-to-tell intermediate cases of only slight
 CM>aberration? No -- they don't need to wear specs!

But it is even more apparent if the lenses are held further away and especially
if they are moved from side to side, slightly.

Leo Bores, M.D.

--  
Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!14!Leo.Bores



Sun, 20 Sep 1992 03:42:11 GMT
 Short sight question

writes:

 EP>things,
 EP>those brains.

Indeed! Aren't you glad you use yours? Don't you wish everybody did!?

Leo Bores, M.D.

--  
Uucp: ...{gatech,ames,rutgers}!ncar!asuvax!stjhmc!14!Leo.Bores



Sun, 20 Sep 1992 03:42:41 GMT
 
 [ 7 post ] 

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