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Home > Archive > Vision > June 2006 > eye glass question: how does Y diopters map to 20/XXX
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eye glass question: how does Y diopters map to 20/XXX
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| 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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| otisbrown@pa.net 2006-06-18, 9:25 pm |
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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:
> 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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| Mike Tyner 2006-06-18, 9:25 pm |
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"Frank" <franklopez2000@yahoo.com> wrote
> 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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| Mike Tyner 2006-06-18, 9:25 pm |
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<otisbrown@pa.net> wrote
> 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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| tkopan1@yahoo.com 2006-06-19, 4:28 pm |
| 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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| A Lieberman 2006-06-19, 4:28 pm |
| 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
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| Dragon28 2006-06-20, 8:24 am |
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A Lieberman wrote:
> On 18 Jun 2006 16:10:41 -0700, otisbrown@pa.net wrote:
>
>
> 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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| otisbrown@pa.net 2006-06-20, 4:26 pm |
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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:
> A Lieberman wrote:
>
> 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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| odisbrown@pa.net 2006-06-20, 4:26 pm |
| 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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| A Lieberman 2006-06-20, 9:25 pm |
| On 20 Jun 2006 07:21:57 -0700, otisbrown@pa.net wrote:
> 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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