Author Topic: Relationship between Snellen Visual acuity and refractive state.  (Read 5776 times)

Offline OtisBrown

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This is a very rough simulation.  But is a start.

http://www.billauer.co.il/simulator.html

There is an error for a positive lens.  If your refractive state is positive, by +1/2 diopter, you will
still have 20/20 vision.  There is no "equation" that will show this.

The ODs always over-prescribe by -1 diopter.  This fact must be understood.  The only way
you get an accurate measurement, is to get your own test lenses, and measure yourself.

The best idea?  Set up your own Snellen, get your own lenses, and check yourself.

PERSONAL NOTE: 
I had 20/20 on my home Snellen, with refractive STATE of (very valuable) +1/2 diotper.

When I HAD to go to the OD, I pushed away the "Phoropter", and read the 20/20 line for them (3/4 the letters).

So what did the "tech" do.  She put the "Phoropter" back in front of my eyes, and insisted that she HAD
to "check for prescription". 

I had no choice but to comply.  So, when she asked a question, is one better, is two better. (for astig and refraction), I
just said, that I saw no difference.

But I HAD to make certain I had vision in the range of 20/25 to 20/20, to do that.


« Last Edit: January 26, 2015, 09:05:11 PM by OtisBrown »

Offline CapitalPrince

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #1 on: January 26, 2015, 08:57:48 PM »
using the 20/20 room lighting (not sunlight). bright snellen guideline
-0.25 about 20/25-20/30
-0.5 about 20/40
-1D about 20/60 20/70

an optometrist who prescribes for max visual acuity in a dark room would look like

borderline 20/20, OD will prescribe -1D with cyl
20/40, probably -1.5D to -2.0D with cyl
20/60 to 20/70 probably -2.5D to -3D with cyl


Offline Alex_Myopic

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #2 on: January 27, 2015, 05:29:53 AM »
My father went to an OD today because I suspected cataract (early stages).
He has 20/25 vision and got prescribed glasses! He was also told to buy them from a specific "expert" eyeglasses shop nearby...  :-[
I will report what prescription he got for 20/25 vision when I'll see it. Told him not to buy... 20/25 above 70 years old is good vision.

Offline gekonus

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #3 on: January 27, 2015, 05:56:30 AM »
using the 20/20 room lighting (not sunlight). bright snellen guideline
-0.25 about 20/25-20/30
-0.5 about 20/40
-1D about 20/60 20/70

an optometrist who prescribes for max visual acuity in a dark room would look like

borderline 20/20, OD will prescribe -1D with cyl
20/40, probably -1.5D to -2.0D with cyl
20/60 to 20/70 probably -2.5D to -3D with cyl

Yeah pretty much that ^^^^ -1D is pretty bad in indoor lights.. pretty much around 20/60

Offline OtisBrown

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #4 on: January 27, 2015, 08:13:53 AM »
Hi Gekonus,

I think a person is very lucky, if he puts up a bright Snellen, and can read 1/2 the letters on the 20/40 line.  That is a good
starting point.

This is because it is an OBJECTIVE measurement.  The other "part" of this issue is to obtain two lenses of -1/2 and -1 diopters,
and use them to determine the minimum minus, to read the 20/20 line.  But this is your issue, I believe.

http://frauenfeldclinic.com/arent-getting-results/

I personally do not claim, "success", because I know it takes long-term wearing of a preventive-plus, to
begin to see results, from a consistent Snellen.

Yes, vision of 20/25 is very good, and at that point you can stop wearing a minus lens - which
would be my major goal in this process.

The hardest fact and issue to face, is that it will take about six to nine months of persistent wearing of a +1.5 diopter at 20 inches, to begin to show objective change in your refractive state, and the line you read on a bright Snellen chart. 

This is why success is always personal.  It just takes too much personal resolve - to do the job properly.

using the 20/20 room lighting (not sunlight). bright snellen guideline
-0.25 about 20/25-20/30
-0.5 about 20/40
-1D about 20/60 20/70

an optometrist who prescribes for max visual acuity in a dark room would look like

borderline 20/20, OD will prescribe -1D with cyl
20/40, probably -1.5D to -2.0D with cyl
20/60 to 20/70 probably -2.5D to -3D with cyl

Yeah pretty much that ^^^^ -1D is pretty bad in indoor lights.. pretty much around 20/60
« Last Edit: January 27, 2015, 11:00:09 AM by OtisBrown »

Offline Alex_Myopic

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #5 on: January 27, 2015, 11:02:25 AM »
Finally the reason why my father became 20/25 while he could not see 20/20 with plus or minus as I had tested him was astigmatism! He didn't have cataract as the OD examined him. He prescribed him -1 cyl and -0.25D.

Offline OtisBrown

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #6 on: January 28, 2015, 09:54:56 AM »

Hi Alex,

You got a different result that I did - in this video.

https://www.youtube.com/watch?v=K7cU-0onSvI

When I hold up a -1 diopter, that can clear my 20/20 line for me.  That is what the OD did for your father, since most of his
prescription was for myopia.

That is why I make this measurement myself.  It saves me getting into and argument with an optometrist.

Finally the reason why my father became 20/25 while he could not see 20/20 with plus or minus as I had tested him was astigmatism! He didn't have cataract as the OD examined him. He prescribed him -1 cyl and -0.25D.

Offline Alex_Myopic

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #7 on: January 28, 2015, 11:42:16 AM »
Hi Mr Brown.
 I agree with you that when 20/40 a person can start plus and never wear minus (except when putting himself or others in dander). But in the case of my father who cannot rehabilitate, should he wear his prescription? I personally believe no, since with 20/25 he is quite good and with his first myopia and astigmatism glasses he can go to a vicious cycle as CapitalPrince and others posted. My question is of course open to everyone.
Mr Brown gives his real name so I hope my question and his answer will not give rise for OD's to legally blame him.

If someone cannot use the Snellen chart by his own a doctor's prescription even for -0.5D might intimidate him/her and never start plus.
« Last Edit: January 28, 2015, 11:44:06 AM by Alex_Myopic »

Offline OtisBrown

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #8 on: January 28, 2015, 12:01:24 PM »
Hi Alex,

I am certain that "patients" are indeed intimidated by the powerful OD.  The "patient" is in fear of asking any questions.
When he looks through the tiny hole of the "Phoropter" he has NO IDEA about what is going on.

The OD, simply ASSUMES that you want you vision made EXTREMELY SHARP.  Thus, people with excellent 20/25 vision,
get a -1.125 (spherical equivalent), when they pass the DMV requirement.

This is why I do this checking at home.

Your father has the benefit of your education, and the ability do to your own independent checking - yourself.

I know you "favor" Bates's concept, and I think this was indeed part of Bates' OBJECTION.  People with
excellent 20/25 vision, were prescribed a -1.5 to -2 diopter lens, AFTER Bate's checked, and their
vision was on the order of 20/20 to 20/25.

But because the person did not "know" about this problem, they NEVER CHECKED themselves.

I know there is some "compromise" here, and it would be wise for the person to be educated about this
over-prescription policy, but everyone is intimidated, and few people
have the ability to check themselves.

The ODs are afraid of the "other" ODs.  They are afraid of their "social arrangements".  They are
afraid to 'stand up', and put their title of "doctor" at risk.  They are afraid of the ignorance of the
public, who "might find out" how it is wrong to put a child into a -1.5 diopter, when the
child passes the 20/25 line.

These are terrible intellectual problems, that we need to face.   But at least we can understand them,
even if we choose to nothing about them.

Hi Mr Brown.
 I agree with you that when 20/40 a person can start plus and never wear minus (except when putting himself or others in dander). But in the case of my father who cannot rehabilitate, should he wear his prescription? I personally believe no, since with 20/25 he is quite good and with his first myopia and astigmatism glasses he can go to a vicious cycle as CapitalPrince and others posted. My question is of course open to everyone.
Mr Brown gives his real name so I hope my question and his answer will not give rise for OD's to legally blame him.

If someone cannot use the Snellen chart by his own a doctor's prescription even for -0.5D might intimidate him/her and never start plus.
« Last Edit: January 28, 2015, 04:37:57 PM by OtisBrown »

Offline CapitalPrince

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #9 on: January 28, 2015, 01:16:31 PM »
-0.25D, -0.5D for a person with low myopia is a HUGE difference.

20/25 is good in room lighting, there is no need for a minus, maybe perhaps a -0.25D night driving in a foreign place.

the astigmatism cylinder should be BANNED. As almost cyl greater than -1.5D is lens induced.

Offline HansK

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #10 on: January 28, 2015, 11:57:31 PM »
With my far-point measurement, I get that I am myopic about -1.3 diopters (left eye) and -1.6 diopters (right eye).

With -1 diopters, I can read further than 20/20, 20/18 to 20/15. According to Otis Brown, I would have a "positive refractive state", which is obviously not true because distant objects are still blurry.

What do you think about it? Does 20/20 mean that you are "emmetropic" (refractive state of 0.0 diopters)? I say no.

If the edge of focus is at 50 cm with +2 reading glasses, then you have a refractive state of 0.0 diopters and you are emmetropic.

Kind regards,
Hans

Offline jimboston

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #11 on: January 29, 2015, 02:50:06 AM »
What do you think about it? Does 20/20 mean that you are "emmetropic" (refractive state of 0.0 diopters)? I say no.

If the edge of focus is at 50 cm with +2 reading glasses, then you have a refractive state of 0.0 diopters and you are emmetropic.

Children and young adults that are considered emmetropic are generally slightly on the positive side (~ +0.5D), which allows them to see 20/15 - 20/10. Myopes, however, begin to drift away from this state quite early in their lives. This is left unnoticed until refraction worsens noticeably. Studies classify myopia as -0.75D.
« Last Edit: August 10, 2015, 11:51:47 PM by jimboston »

Offline OtisBrown

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #12 on: January 29, 2015, 01:06:34 PM »

Hi Hans,

If you define a requirement that everyone must have 20/10 vision to be emmetropia-normal, then you have virtually made everyone who has 20/20 vision - "not normal".

I have posted some remarks on this issue.  You need your refractive STATE to change by +1.25 diopters, in order to see 20/10 vision.  That is fine and excellent. How you measure and define your success, (to get to 20/10 vision) of course is totally up to you.

I think that most of us understand that if we read most of the letters on the 20/20 line, our vision is in fact normal.


With my far-point measurement, I get that I am myopic about -1.3 diopters (left eye) and -1.6 diopters (right eye).

With -1 diopters, I can read further than 20/20, 20/18 to 20/15. According to Otis Brown, I would have a "positive refractive state", which is obviously not true because distant objects are still blurry.

What do you think about it? Does 20/20 mean that you are "emmetropic" (refractive state of 0.0 diopters)? I say no.

If the edge of focus is at 50 cm with +2 reading glasses, then you have a refractive state of 0.0 diopters and you are emmetropic.

Kind regards,
Hans

Offline OtisBrown

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #13 on: January 29, 2015, 01:14:59 PM »

Prolonged close work, results in the normal eye, changing its refractive STATE, from "plus" to minus".  This is now recognized in law.

http://frauenfeldclinic.com/taiwan-passes-law-limit-child-gaming/

It would be wise, to "end" the natural eye's responsiveness to "long-term" near, not by passing a new law, but by wearing a plus for all close work,
and measuring your refractive STATE yourself.

Offline HansK

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Re: Relationship between Snellen Visual acuity and refractive state.
« Reply #14 on: January 29, 2015, 06:48:32 PM »
Hi Mr. Otis,

I do not say 20/10 is emmetropia. I do not care about the Snellen score. Emmetropia is - for me - that your edge of focus with +2 reading glasses is at 50 cm.

What the retina is able to do - that is another thing. And I do not say if you are nearsighted (-0.5 diopters and more) you have to wear glasses. Far-point measurements are very accurate to get your refractive state and not the 20/20 line.

The 20/20 line is a "guideline", but as I have said - with undercorrection, I can read further than 20/20. Do I have a positive refractive state? No, of course - I do not have. I can do better, but achieving this with a minus is DANGEROUS, while achieving it with naked eyes would be awesome.

Do you now understand what I mean?

Kind regards,
Hans