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Forum index » Medicine forums » vision
eye glass question: how does Y diopters map to 20/XXX
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A Lieberman
medicine forum Guru Wannabe


Joined: 30 Apr 2005
Posts: 194

PostPosted: Wed Jun 21, 2006 12:57 am    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
On 20 Jun 2006 07:21:57 -0700, otisbrown@pa.net wrote:

Quote:
Dear Dragon,

I said APPROXIMATE.

Dear Dragon,

Please disregard Otis's postings. He is not in the medical profession nor
in any position to give medical advice.

Thanks!

Allen
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Odis S. Brown
medicine forum beginner


Joined: 17 Apr 2006
Posts: 32

PostPosted: Tue Jun 20, 2006 6:17 pm    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
Dear Dragon,

Subject: off the meds again.

Obviously, I have stopped taking my medication
because of potential complex interactions with
the "ECT."

Please be gentle with me in the coming weeks.
I may tend toward incoherence, irrationality, and
the random spewing of nonsense.

Best,

Odis
At Least I'm Finally Outpatient
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otisbrown@pa.net
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1447

PostPosted: Tue Jun 20, 2006 2:21 pm    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
Dear Dragon,

I said APPROXIMATE.

I said NO FORMULA.

But, the correct method would be to use
a trial lens FOR THAT PERSON.

Thus there can be no formula.

Best,

Otis



Dragon28 wrote:
Quote:
A Lieberman wrote:
On 18 Jun 2006 16:10:41 -0700, otisbrown@pa.net wrote:

Dear Frank,

There is no exact "formula".

Dear Frank,

Please disregard Otis's postings. He is not in the medical profession and
not in any position to give medical advice.

Thanks!

Allen

I've always thought that there is no formula for that.
I have a friend with -2 who sees 20\200 uncorrected... he's not -4.
Strange.
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Dragon28
medicine forum beginner


Joined: 19 Feb 2006
Posts: 24

PostPosted: Tue Jun 20, 2006 12:27 pm    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
A Lieberman wrote:
Quote:
On 18 Jun 2006 16:10:41 -0700, otisbrown@pa.net wrote:

Dear Frank,

There is no exact "formula".

Dear Frank,

Please disregard Otis's postings. He is not in the medical profession and
not in any position to give medical advice.

Thanks!

Allen

I've always thought that there is no formula for that.
I have a friend with -2 who sees 20\200 uncorrected... he's not -4.
Strange.
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A Lieberman
medicine forum Guru Wannabe


Joined: 30 Apr 2005
Posts: 194

PostPosted: Mon Jun 19, 2006 8:59 pm    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
On 18 Jun 2006 16:10:41 -0700, otisbrown@pa.net wrote:

Quote:
Dear Frank,

There is no exact "formula".

Dear Frank,

Please disregard Otis's postings. He is not in the medical profession and
not in any position to give medical advice.

Thanks!

Allen
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tkopan1@yahoo.com
medicine forum beginner


Joined: 05 Apr 2006
Posts: 13

PostPosted: Mon Jun 19, 2006 6:07 pm    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
There are actual, many variables such as lighing and how large or small
the pupil is at the time of testing. The smaller the pupil, the
greater the clarity to a point due to increasing depth of field.

--Dr. Tom
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Mike Tyner
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1299

PostPosted: Mon Jun 19, 2006 12:53 am    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
<otisbrown@pa.net> wrote

Quote:
and using a trial-lens kit, find the weakest
minus that would clear the 20/20 line.

Why would he want blurry vision?

"O'Leary and his colleagues at the National University of Malaysia in Kuala
Lumpur, undercorrected the sight of half the children and fully corrected
the rest. Then they measured the length of the eyeball with ultrasound every
six months. To their surprise, they found that the eyeball elongates faster
when vision is undercorrected."

-New Scientist, Nov 2002
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Mike Tyner
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1299

PostPosted: Mon Jun 19, 2006 12:46 am    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
"Frank" <franklopez2000@yahoo.com> wrote

Quote:
If an eye requires a spherical correction of -4.00 diopters, what
distance does that map to (XXX) as compared to what a person with
perfect eyes can see at 20 feet?

For example, is it 20/200? 20/300?

Is there a formula for calculating 20/XXX based on Y diopters?

There is a formula, but it has problems.

IF you mean just myopes with no astigmatism, it's still a scattered plot and
scattered plots are notoriously unreliable.

For a best-fit formula, a reasonable attempt would be

0.43 x D = log(20/xxx)

That's from a 1997 thread here on s.m.v.

Someone with -400 diopters of pure myopia isn't likely to see better than
20/200, in my exam room, without squinting.

With squinting, or bright light, they might see 20/40 or better, and 20/25
through a pinhole.

-MT, OD
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otisbrown@pa.net
medicine forum Guru


Joined: 24 Mar 2005
Posts: 1447

PostPosted: Sun Jun 18, 2006 11:10 pm    Post subject: Re: eye glass question: how does Y diopters map to 20/XXX Reply with quote
Dear Frank,

There is no exact "formula".

But a rough "calculation" suggest that it takes
about -1 diopter to clear the 20/20 line if
a person has naked-eye visual-acuity of 20/70.

So, using you example, -4 * 70 = 20/280 visual acuity.

Of course, you could have the person read the chart at
20/280, and using a trial-lens kit, find the weakest
minus that would clear the 20/20 line.

Hope this clarifies this issue for you.

Best,

Otis



Frank wrote:
Quote:
If an eye requires a spherical correction of -4.00 diopters, what
distance does that map to (XXX) as compared to what a person with
perfect eyes can see at 20 feet?

For example, is it 20/200? 20/300?

Is there a formula for calculating 20/XXX based on Y diopters?
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Frank
medicine forum beginner


Joined: 02 Aug 2005
Posts: 7

PostPosted: Sun Jun 18, 2006 10:45 pm    Post subject: eye glass question: how does Y diopters map to 20/XXX Reply with quote
If an eye requires a spherical correction of -4.00 diopters, what
distance does that map to (XXX) as compared to what a person with
perfect eyes can see at 20 feet?

For example, is it 20/200? 20/300?

Is there a formula for calculating 20/XXX based on Y diopters?
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Google
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