Why 20 feet?

[XMotorXRacer]

I can look at a chart at 20ft, using -.25 lenses don't make a bit of a
difference. When I look at objects 500ft away, things are a 'hair' bit shaper
with the -.25 lenses.

I would think that when given the subjective choices during an exam ( A better
than B? ) this would fine tune the vision for 20ft.

Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

[Mark A]

Post by XMotorXRacer
I can look at a chart at 20ft, using -.25 lenses don't make a bit of a
difference. When I look at objects 500ft away, things are a 'hair' bit shaper
with the -.25 lenses.
I would think that when given the subjective choices during an exam ( A better
than B? ) this would fine tune the vision for 20ft.
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

The main reason 20 ft is used is because of logistical reasons. To have a
chart 50 or 100 feet away would require facilities that many don't have and
the chart would have to be much larger. In theory it does not make much
difference, but the Rx could be different in some cases. Many optometrists
use mirrors just to get to 20 feet.

[Repeating Rifle]

Post by XMotorXRacer
I can look at a chart at 20ft, using -.25 lenses don't make a bit of a
difference. When I look at objects 500ft away, things are a 'hair' bit shaper
with the -.25 lenses.
I would think that when given the subjective choices during an exam ( A better
than B? ) this would fine tune the vision for 20ft.
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

I have estimated that the actual distance to the test chart image is usually
less than 20 feet even with the use of mirrors. 20 feet is about 6 meters.
Thus the lens power should be about +0.16D to convert 20 feet infinity
optically to infinity. For a shorter distance, the 0.25D would be just about
right. I would expect anyone with even a modicum of accommodation would not
know the difference.

Bill

[LarryDoc]

Post by XMotorXRacer
I can look at a chart at 20ft, using -.25 lenses don't make a bit of a
difference. When I look at objects 500ft away, things are a 'hair' bit shaper
with the -.25 lenses.
I would think that when given the subjective choices during an exam ( A better
than B? ) this would fine tune the vision for 20ft.
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

As I recall the science of this:

20 feet, 6.3 meters, is considered to be the distance at which an
emmetropic (no optical correction) will be at rest, zero focus (no
accomodation), zero parallax error (both eyes parallel) , and therefore
optical infinity. It is partially based on the assumption of an average
63mm distance between the optical axis of the two eyes.

But all is not so mathematical. It appears that we can sometimes measure
a residual .12 diopters of error at the theoretical point of infinity. I
don't recall exactly why that is or if that is still accepted, but it
doesn't really matter in real life. One explanation was that the human
optical system is never completely spherical (and therefore is at least
somewhat abberated) or completely at rest. Makes sense to me.

The reason why adding -.25 *sometimes* appears to make things at very
far distances look "a hair sharper" has more to do with adjustments made
to the various optical abberrations within the system. It should not
matter if the testing distance is 6.3 meters, or 630 meters. When the
optical system is "overcorrected", it stimulates a neuro-motor response
that causes pupillary constriction. The smaller aperture reduces
certain abberations and creates a longer depth of focus.

-.25 (or more) more power also sometimes creates the appearance of
better acuity at night and this is due to a different phenomena --- the
empty-space or night-myopia scenario. It seems as if the human optical
system assumes that the world is closer at night. If there is nothing
to lock focus at a distance beyond 6 meters, the system "assumes" a
resting point somewhat nearer.

--LB

[Robert Redelmeier]

Post by LarryDoc
-.25 (or more) more power also sometimes creates the
appearance of better acuity at night and this is due to
a different phenomena --- the empty-space or night-myopia
scenario. It seems as if the human optical system assumes
that the world is closer at night. If there is nothing
to lock focus at a distance beyond 6 meters, the system
"assumes" a resting point somewhat nearer.

I hadn't heard this explanation.

I'd always thought that miss-corrected vision was worse
at night due to open iris effects (reduced aperature
defraction effect and perhaps worse peripheral lens)

-- Robert

[Otis Brown]

Dear Larry and friends,

About 100 years ago, a scientist noticed that
a minus lens of about -1 diopter made his
deep-dusk vision clearer.

As you said, this is called "tonic accommodaiton".

When the retina can not sense "micro-blur", the
system moves negative by betwee zero to minus 4 diopters.


Best,

Otis
Engineer

Post by LarryDoc

Post by XMotorXRacer
I can look at a chart at 20ft, using -.25 lenses don't make a bit of a
difference. When I look at objects 500ft away, things are a 'hair' bit shaper
with the -.25 lenses.
I would think that when given the subjective choices during an exam ( A better
than B? ) this would fine tune the vision for 20ft.
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

20 feet, 6.3 meters, is considered to be the distance at which an
emmetropic (no optical correction) will be at rest, zero focus (no
accomodation), zero parallax error (both eyes parallel) , and therefore
optical infinity. It is partially based on the assumption of an average
63mm distance between the optical axis of the two eyes.
But all is not so mathematical. It appears that we can sometimes measure
a residual .12 diopters of error at the theoretical point of infinity. I
don't recall exactly why that is or if that is still accepted, but it
doesn't really matter in real life. One explanation was that the human
optical system is never completely spherical (and therefore is at least
somewhat abberated) or completely at rest. Makes sense to me.
The reason why adding -.25 *sometimes* appears to make things at very
far distances look "a hair sharper" has more to do with adjustments made
to the various optical abberrations within the system. It should not
matter if the testing distance is 6.3 meters, or 630 meters. When the
optical system is "overcorrected", it stimulates a neuro-motor response
that causes pupillary constriction. The smaller aperture reduces
certain abberations and creates a longer depth of focus.
-.25 (or more) more power also sometimes creates the appearance of
better acuity at night and this is due to a different phenomena --- the
empty-space or night-myopia scenario. It seems as if the human optical
system assumes that the world is closer at night. If there is nothing
to lock focus at a distance beyond 6 meters, the system "assumes" a
resting point somewhat nearer.
--LB

[LarryDoc]

Post by Otis Brown
Dear Larry and friends,
About 100 years ago, a scientist noticed that
a minus lens of about -1 diopter made his
deep-dusk vision clearer.

Duh.

Post by Otis Brown
As you said, this is called "tonic accommodaiton".

Never used those words. Duh.

Post by Otis Brown
When the retina can not sense "micro-blur",

What's that? A new term from Otis the term-maker?

Post by Otis Brown
the system moves negative by betwee zero to minus 4 diopters.

Who said? You? Can you prove it? Of course not.

LB

[andrew Judd]

Post by LarryDoc

Post by Otis Brown
Dear Larry and friends,

Otis seemed to be referring to LD's comment

Post by LarryDoc

Post by Otis Brown

Post by LarryDoc
It seems as if the human optical

system assumes that the world is closer at night. If there is nothing
to lock focus at a distance beyond 6 meters, the system "assumes" a
resting point somewhat nearer.

But Larry's comment is a bit confusing because he says "the world is
closer at night"

This effect is also possible when looking at brightly lit none
visually compelling stimuli such as blue sky or uniform grey cloud.

Should we give Larry a good kicking? Probably not, his meaning was
clear.

Post by LarryDoc

Post by Otis Brown
About 100 years ago, a scientist noticed that
a minus lens of about -1 diopter made his
deep-dusk vision clearer.

Duh.

Post by Otis Brown
As you said, this is called "tonic accommodaiton".

Never used those words. Duh.

These words were not used but they were implied, since tonic
accommodation is another term for dark focus of accommodation, or the
eyes resting state when a visually compelling stimulus is absent.

But Otis is getting confused by the confusing "world is closer at
night", assumes that night myopia is being mentioned, goes off on a
tangent and gets a good kicking:-(

Post by LarryDoc

Post by Otis Brown
When the retina can not sense "micro-blur",

What's that? A new term from Otis the term-maker?

Post by Otis Brown
the system moves negative by betwee zero to minus 4 diopters.

Well once again because Otis the term-maker is creative in his use of
language and uses "micro-blur" rather than "absence of a visually
compelling stimulus" he gets another good kicking:-(

Post by LarryDoc
Who said? You? Can you prove it? Of course not.

But in this case apart from his confusing language he is quite
correct.

When the human eye cannot sense a visually compelling stimulus it
moves between zero and a minus amount to rest at the dark focus of
accomodation for that individual.

[Otis Brown]

Dear Friends,

Subject: The retina's detection of "micro-blur".

Micro blur means that the blur exceeds the optical dead-band.

Micro-blur is below your level of preception.

If these small "error corrections" did not exist, the
lens would not be able to "correct" for changes in
target movement.

The paper discussing "tonic accommodation" (referenced) is
posted in the paper listed
in "i-see", by Alex Eulenberg.

http://www.i-see.org/otis_brown/cybernetic_model.html

The model follows a concept of "reverse engineering"
the eye, to build a "working model" of "mechanical accommodation".

To clarify, I suggest a very simple model of the retina
(for clarity).

Most of the experimental evidence comes from published
papers on the use of an infrared optometer.

To clarify certain issues I move the lens back-and-forth,
(yes I know, the lense is moved by muscles) again
to get the concept clearly across.

The model is for clarity. The mathematical model
is very close to the model produced by John Semmlow
and Georg Hung.

But enjoy, that is why we talk here.

Best,

Otis
Engineer

[LarryDoc]

Post by Otis Brown
Subject: The retina's detection of "micro-blur".
Micro blur means that the blur exceeds the optical dead-band.

And here you go again, Otis. Dead-band. Yet ANOTHER made-up word that
goes so well with the previous made-up word "micro-blur". We're so glad
you're able to provide made-up definitions for your made-up terms.

Post by Otis Brown
The paper discussing "tonic accommodation" (referenced) is
posted in the paper listed
in "i-see", by Alex Eulenberg.

Yet another quack who previously post here but was convinced to leave
after most of us provided adequate evidence of his quackery. Perhaps
YOU can LEARN something from him.

Post by Otis Brown
But enjoy, that is why we talk here.

Do try to remember the charter of this newsgroup: it's under the SCI
heading. That means "science", not made-up terms, made-up definitions
and invented theories based on other people's quackery.

Got it?

--LB

[David Robins, MD]

Well, LarryDoc, I'm not so sure Otis is dead wrong about the dead-band.

While I admit I don't know the exact physiology of the focus system, I can
speak to the control-system feedback-loop of a typical system. In a control
system, there has to be a tolerance level under which the feedback system
will not respond, otherwise it will oscillate all the time. This is what
determines the hysteresis of the system. It sounds like Otis is talking
about such a dead-band, or hysteresis zone, which I assume exists in the
human focus system also. The detection of being past the edge of the
hysteresis zone would be the earliest detection of blur, which I guess
someone (Otis) could call a ³micro-blur². This would be the control-systems
engineer part of Otis¹ background speaking.

Other than that, I can¹t comment on the possibility of made-up words.



On 7/3/04 12:59 PM, in article

Post by LarryDoc

Post by Otis Brown
Subject: The retina's detection of "micro-blur".
Micro blur means that the blur exceeds the optical dead-band.

And here you go again, Otis. Dead-band. Yet ANOTHER made-up word that
goes so well with the previous made-up word "micro-blur". We're so glad
you're able to provide made-up definitions for your made-up terms.

Post by Otis Brown
The paper discussing "tonic accommodation" (referenced) is
posted in the paper listed
in "i-see", by Alex Eulenberg.

Yet another quack who previously post here but was convinced to leave
after most of us provided adequate evidence of his quackery. Perhaps
YOU can LEARN something from him.

Post by Otis Brown
But enjoy, that is why we talk here.

Do try to remember the charter of this newsgroup: it's under the SCI
heading. That means "science", not made-up terms, made-up definitions
and invented theories based on other people's quackery.
Got it?
--LB

[Robert Redelmeier]

Post by David Robins, MD
Well, LarryDoc, I'm not so sure Otis is dead wrong about
the dead-band.
While I admit I don't know the exact physiology of the focus
system, I can speak to the control-system feedback-loop of
a typical system. In a control system, there has to be a
tolerance level under which the feedback system will not
respond, otherwise it will oscillate all the time. This is

There is also the optical concept of circle-of-confusion,
somewhat related to the finite size of focal-plane
receptors (rods, cones, silver crystals or transistors).
The system doesn't need to focus finer than this, and it
also produces "depth of field" effects very troublesome
for anyone using a microscope.

-- Robert

[Francine]

Post by Robert Redelmeier

Post by David Robins, MD
While I admit I don't know the exact physiology of the focus
system, I can speak to the control-system feedback-loop of
a typical system. In a control system, there has to be a
tolerance level under which the feedback system will not
respond, otherwise it will oscillate all the time. This is

There is also the optical concept of circle-of-confusion,
somewhat related to the finite size of focal-plane
receptors (rods, cones, silver crystals or transistors).
The system doesn't need to focus finer than this, and it
also produces "depth of field" effects very troublesome
for anyone using a microscope.
-- Robert

The visual system may need extremely fine focus if one is a jeweler. jewelry
model maker needs to resolve distances of 1/20 of a millimeter in order to
do his job correctly. He typically wears a magnifying device called an
optivisor in to be able to see this clear at near, if they need
it.Optivisors come in strengths of 1, 5, 7, and 10. An optivisor #10, I
believe, magnifies 350% and has prisms built into it as well. It's focal
length is about 3 or 4 inches. Some jewelers can't wear it, as it can cause
discomfort to the eyes of some people.

Cheers,
Francine

[Otis Brown]

Post by David Robins, MD
Well, LarryDoc, I'm not so sure Otis is dead wrong about the dead-band.
While I admit I don't know the exact physiology of the focus system, I can
speak to the control-system feedback-loop of a typical system. In a control
system, there has to be a tolerance level under which the feedback system
will not respond, otherwise it will oscillate all the time. This is what
determines the hysteresis of the system. It sounds like Otis is talking
about such a dead-band, or hysteresis zone, which I assume exists in the
human focus system also. The detection of being past the edge of the
hysteresis zone would be the earliest detection of blur, which I guess
someone (Otis) could call a ³micro-blur². This would be the control-systems
engineer part of Otis¹ background speaking.
Other than that, I can¹t comment on the possibility of made-up words.

*********

Dear David,

Thanks for the commentary.

I have been very respectful to medical doctors. You deal
with massive disasters of the human body -- which I could
never deal with.

In regards to the ODs on this site -- they think I am
"critical". In fact I am not. I can never deal
with a great mass of people walking in off the
street -- demanding a "quick fix" (it would seem)
and nothing else.

It is just that I expect a considerable degree
of self-motivation in the person I talk to.

And I assume the person can become intelligent
about the issues involved.

The OD simply does not have the time to go through
these issues, nor would I expect him to.

But for an engineer, who understands optical dead-band
(variously called depth-of-field, depth-of-focus), then
I expect an effective review of the accommodation
system.

Such a review MUST take into account the detection
of blur (or micro-blur) at the surface of the
retina, and must account for corresponding
micro-changes in the power (or position) of
the lens of the eye.

That is the purpose of building an
"engineering model" of the accommodation system -- that
is different that the "tradigional" Donders-Helmholtz
model and concept.

And thanks for your review. Incidentally, Dr. Dave
Guyton worked with us (he is skeptical I am sure) without
ANY RECOGNITION AND OR REWARD. And his time
was and is very valueable indeed.

Let this statement honor him as a friend of engineers and people
who choose to fight "impossible battles".

Best,

Otis
Engineer

[Dr. Leukoma]

Post by David Robins, MD
Well, LarryDoc, I'm not so sure Otis is dead wrong about the
dead-band.

Even a stopped clock tells the correct time twice a day.

DrG

[Otis Brown]

Dear Larry-Doc,

Talking about organic defects (refractive errors) OK, you
are the expert.

Talk about engineering design issues -- and you
are a layman.

The term "dead-band" is common in both electrical,
optical and mechanical engineering.

It means a period of time where the "input" can be
varied -- and the output remains with no change.

You will find the term used by Lawrence Stark
George Hung and John Semmlow.

Post by LarryDoc

Post by Otis Brown
Subject: The retina's detection of "micro-blur".
Micro blur means that the blur exceeds the optical dead-band.

And here you go again, Otis. Dead-band. Yet ANOTHER made-up word that
goes so well with the previous made-up word "micro-blur".

I used the term "micro-blur" to describe the need of the
retina to detect "blur" so as to drive the lens back
into the dead-band. This action (at about 4 to 1/4 herrtz
creates an "oscillation" that is common to control-systems of
this design.

Any engineer designing a control system (similar to
the accommodation system) that must actually WORK,
will understand this specific design-problem.

Sorry you do not understand the concept of "dead-band"
but I understand you are an optometrist, and not
an engineer.


We're so glad you're able to provide made-up definitions
for your made-up terms.

In your ignorance of control-system I understand
you are going to state (with no proof) that
dead-band is a "made-up" term. Engineering
is not your profession.

Post by LarryDoc

Post by Otis Brown
The paper discussing "tonic accommodation" (referenced) is
posted in the paper listed
in "i-see", by Alex Eulenberg.

Yet another quack who previously post here but was convinced to leave
after most of us provided adequate evidence of his quackery. Perhaps
YOU can LEARN something from him.

As always you spew "quack" aroung a lot. It is a
substitute for desciplined thinking about the
behavior of the natural eye as a sophisticated
system. Alex is a good man.

Post by LarryDoc

Post by Otis Brown
But enjoy, that is why we talk here.

Do try to remember the charter of this newsgroup: it's under the SCI
heading.

Science is different from medicine -- in the sense that
you determine the exact behavior of the natural
eye BEFORE you EVER use the concept of DEFECT.

But again you are "medical" and not an engineer.

But do not presume you know all that much about
the dynamic behavior of a sophisticated control-system
my friend.


That means "science", not made-up terms, made-up definitions
and invented theories based on other people's quackery.

You do indeed have a very limited vision -- that supressess
new concepts. But that just makes
our convesations more interesting.

Post by LarryDoc
Got it?

Yes I "got it", that you have no clue about
the behavior of the natural eye as
a superior physiological system that
controls its refractive status to
its average visual enviroment (in diopters).

Best,

Otis
Engineer

[andrew Judd]

Otis

Post by Otis Brown
the behavior of the natural eye as
a superior physiological system that
controls its refractive status to
its average visual enviroment (in diopters).

Emmetropic eyes do appear to be 'driven' to maintain emmetropia so
that emmetropia is not simply a chance event in the population of all
eyes.

However in myopia something else is happening. Since you are
principally concerned with myopic eyes you must address this.

Andrew

[Otis Brown]

Dear Friends,


Subject: Some commentary on the origin of the term 20/20 and what
it actually means. How to calculate the size
letters and make your own.

From the study of optics and astronomy, it was established
that the "high quality" human eye was able to resolve (or
separate) two stars separated by about 1 minute-of-arc.

If you analyze the eye as an optical device, it can be shown
that (at the wave-length of light) diffraction effects will begin
to "kick in", and prevent resolution of much better that 1 MOA.
Some people can do better than this -- however.

Dr. Snellen decided that there should be a simpler method of
checking the eye. He used "bars" separated by 1' of arc. The
effect of making letters to this standard, was that the letters
were 5 minutes-of-arc high.

To calculate the height of the letters, it is easier to use
the "Radian" angle measurement system.

Thus:

5 minutes-of-angle = 0.001455 Radians

Then:

At 6.1 meters, multiply by 0.001455 to = 0.0089

therefore 20/20 = 0.89 cm letters

In inches, 12 * 20 = 240 inches

240 * 0.001455 = 0.35 inches at 20 feet.

So we should say 20/20/0.35 inches to be clear.

Obviously you can obtain the 20/40 size letter by multiplying

0.35 * 2 = 0.70 inches.

The same holds for 20/80 size characters.

Using the above calculations you can set up a chart for 10
feet, or 3.05 meters. You can easily calculate the size of the
letters and cut them out of the newspaper.


THE VISUAL STANDARD CHANGED DURING WORLD WAR I

The above standard is the sharpest vision that can be
obtained by use of a minus lens. This standard was determined to
be excessively high, and therefore unreasonable. As a result,
during WWI, they realized that a great mass of soldiers would be
wearing minus glasses if this standard were enforced. They then
decided to not use the 20/20 standard, and choose 20/40 as
reasonable and acceptable.

If you are working to achieve a reasonable standard of vision
I suggest using the reasonable 20/40 standard rather than using a
strong minus lens to get to 20/20.

This is to avoid having your eyes "adapt" to the minus lens.
This is a TEMPORARY measure. If you are very persistent
with the plus, you can verify your distant vision clearing
to 20/20 -- if you work very hard at it.

As always, enjoy the conversation.

Best,

Otis
Engineer

[Jan]

Post by Otis Brown
Subject: Some commentary on the origin of the term 20/20 and what
it actually means. How to calculate the size
letters and make your own.
Dr. Snellen decided that there should be a simpler method of
checking the eye. He used "bars" separated by 1' of arc. The
effect of making letters to this standard, was that the letters
were 5 minutes-of-arc high.

Please notice the lines may not exceed the 1' of arc.
They may not exceed the 1/5 of the heigt of the character in thickness.
Very important.
Please be more precise Otis when explaining.

Post by Otis Brown
To calculate the height of the letters, it is easier to use
the "Radian" angle measurement system.
5 minutes-of-angle = 0.001455 Radians
At 6.1 meters, multiply by 0.001455 to = 0.0089
therefore 20/20 = 0.89 cm letters
In inches, 12 * 20 = 240 inches
240 * 0.001455 = 0.35 inches at 20 feet.
So we should say 20/20/0.35 inches to be clear.
Obviously you can obtain the 20/40 size letter by multiplying
0.35 * 2 = 0.70 inches.
The same holds for 20/80 size characters.
Using the above calculations you can set up a chart for 10
feet, or 3.05 meters. You can easily calculate the size of the
letters and cut them out of the newspaper.

The lines (thickness) from which the characters are build up may not exceed
the 1/5 of the height of the character.
Very important Otis.
And please explain the need to accommodate (or not) issue when you get your
chart nearer as 20 feet away.
And how about astigmatism?

Post by Otis Brown
THE VISUAL STANDARD CHANGED DURING WORLD WAR I

Maybe the requirement in vision acuity did changed Otis, not the standard
itself.

Post by Otis Brown
The above standard is the sharpest vision that can be
obtained by use of a minus lens.

There you go again Otis, charlantery language.
It is indeed possible that some one needs a minus correction to achieve this
vision acuity but also it is possible that a minus correction decrease some
ones vision acuity.

This standard was determined to

Post by Otis Brown
be excessively high, and therefore unreasonable.

No, they need cannon-fodder, not a sharp looking human.

As a result,

Post by Otis Brown
during WWI, they realized that a great mass of soldiers would be
wearing minus glasses if this standard were enforced. They then
decided to not use the 20/20 standard, and choose 20/40 as
reasonable and acceptable.

Yes, you have to go for numbers those days Otis, not for sharp shooters.

Post by Otis Brown
If you are working to achieve a reasonable standard of vision
I suggest using the reasonable 20/40 standard rather than using a
strong minus lens to get to 20/20.

Again the charlantery language.
20/40 is 50% of the normal standard vision acuity.
You are not in the position to make your own (rubber)definitions of what is
reasonable Otis,

Post by Otis Brown
This is to avoid having your eyes "adapt" to the minus lens.
This is a TEMPORARY measure. If you are very persistent
with the plus, you can verify your distant vision clearing
to 20/20 -- if you work very hard at it.

Again the charlantery language.
The above is impossible in axial myopia except for those with a high vision
acuity (say 20/10) who wanted there vision acuity to decrease to 20/20.

Jan (normally Dutch spoken)

Post by Otis Brown
Otis
Engineer

[Dr Judy]

Post by Otis Brown
Dear Friends,
THE VISUAL STANDARD CHANGED DURING WORLD WAR I
The above standard is the sharpest vision that can be
obtained by use of a minus lens.

20/20 and Snellen letters have no more to do with " the sharpest vision that
can be obtained by use of a minus lens." than the kilogram has to do with
how much weight a person can lift. Snellen letters are simply standard size
optotypes used for measuring visual acuity, whether the acuity measured
unaided, with a minus lens or with a plus lens.

This standard was determined to

Post by Otis Brown
be excessively high, and therefore unreasonable. As a result,
during WWI, they realized that a great mass of soldiers would be
wearing minus glasses if this standard were enforced. They then
decided to not use the 20/20 standard, and choose 20/40 as
reasonable and acceptable.

This is nonsense. 20/40 was never defined as a reasonable and acceptable
level of vision with the implication that one would not bother to correct
vision to better than that level. It was defined as the minimal safe level
of vision for certain tasks, ie people with vision worse than the minimum
would not be allowed to do the task..

Driver licensing bodies, pilot examiners etc agree that blind people should
not do certain tasks like shoot guns, drive cars or fly planes because they
will endanger themselves and others. They also agree that slightly less
than normal vision would be safe for these tasks. So the question then
becomes, at what point between normal vision (20/20) and blindness should
the safety cut off be? The answer varies with the task and the agency but
driving and flying is usually limited to people who can get at least 20/40
to 20/60 with or without glasses. For some tasks, if there is a
considerable risk of glasses falling off, then the limit may be specified as
without glasses.

Post by Otis Brown
If you are working to achieve a reasonable standard of vision
I suggest using the reasonable 20/40 standard rather than using a
strong minus lens to get to 20/20.

It does not take a strong minus lens to correct a myope from 20/40 to 20/20.

Dr Judy

Post by Otis Brown
This is to avoid having your eyes "adapt" to the minus lens.
This is a TEMPORARY measure. If you are very persistent
with the plus, you can verify your distant vision clearing
to 20/20 -- if you work very hard at it.
As always, enjoy the conversation.
Best,
Otis
Engineer

[Roland Izaac]

Post by XMotorXRacer
I can look at a chart at 20ft, using -.25 lenses don't make a bit of a
difference. When I look at objects 500ft away, things are a 'hair' bit shaper
with the -.25 lenses.
I would think that when given the subjective choices during an exam ( A better
than B? ) this would fine tune the vision for 20ft.
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

20 feet or six meters is used for the following reason

If the refraction is measured at 40 cm, then myopia of -2.5 diopters
will be missed. at 1 meter -1.00 diopters of myopia will be missed.
however when measured at 6 meters, only -0.16 diopters will be missed.
This if less then the -0.25 diopter steps that are used to measure
myopia.

Roland Izaac

[Dan Abel]

Post by XMotorXRacer
difference. When I look at objects 500ft away
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

I'm no expert here, but I seem to remember that the difference between 20'
and infinity was at the limit of measurement. The important thing is that
no OD that I'm aware of has an office 500' long! The testing needs to be
done at consistent light levels, so outside is not good. In fact, most
offices don't have a 20' run either, so mirrors are used. My OD (private
practice), who is a little anal about things, does have an oddly shaped
office with the full distance. A few years back he lost his office. It
was in a building that housed the anchor tenant for the shopping center, a
farm supply store, which most people used as a hardware store. I live in
a suburb of San Francisco, so a farm supply store looks a little odd. The
store closed (just this one out of the chain) and Orchard Supply Hardware
moved in. Evidently they didn't like the building, and it was razed, and
a new one put in, much larger. There was no longer room for my OD and
some other small places. There was room elsewhere in the shopping center,
though. At one visit he told me that he was moving around the corner. A
year later, at my next visit, I was amazed to see that his new office was
absolutely identical, down to the colors and wood trim. When I went into
the exam room, I was even more astonished to see the same odd layout,
which I had always thought was due to circumstances beyond his control. I
commented on how it seemed that his office looked identical to the old
one. He said that it was no coincidence, that a condition of his move was
that they reproduce his office exactly.

[LarryDoc]

Post by Dan Abel
In fact, most
offices don't have a 20' run either, so mirrors are used. My OD (private
practice), who is a little anal about things, does have an oddly shaped
office with the full distance.

Ah....mirrors and smoke, or smoke and mirrors.

OK. I'll have to admit to some of the same harebrain ideas as your doc.
My room is 25' long, 10 to 13' wide. I refract patients with indirect,
balanced spectrum lighting of "medium" intensity and then often do it
again with considerably reduced light levels, both with high contrast
and low contrast distance and near targets. Not always, but especially
if a patient has large pupils, ADD, contrast sensitivity issues (like
cataracts, LASIK'd folk, or other things that need to factored into the
mix.

I do things to try to relate to the environment the eyes are asked to
work in real life.

I even make sure my lenses are clean.

--LB

[David Robins, MD]

20 feet (about 6 meters) is not at the limit of measurement. The
accommodation for that distance is 1/6 meters, which is 1/6 of a diopter.
So, if you refract perfectly at 20 feet, one should add 1/6 of a diopter
more minus to go out to infinity. Lenes typically come in 0.25D increments,
and can be ordered in 0.12D increments (1/8 of a diopter.) ANSI lens
manufacturing standards are about +/1 0.13D iup to 6.50 diopters.

On 7/1/04 11:26 AM, in article

Post by Dan Abel

Post by XMotorXRacer
difference. When I look at objects 500ft away
Just wondering why 20' is so commonly used? Is 20' presumed to be close
enough to infinate distance for focusing of the eyes?

I'm no expert here, but I seem to remember that the difference between 20'
and infinity was at the limit of measurement. The important thing is that
no OD that I'm aware of has an office 500' long! The testing needs to be
done at consistent light levels, so outside is not good. In fact, most
offices don't have a 20' run either, so mirrors are used. My OD (private
practice), who is a little anal about things, does have an oddly shaped
office with the full distance. A few years back he lost his office. It
was in a building that housed the anchor tenant for the shopping center, a
farm supply store, which most people used as a hardware store. I live in
a suburb of San Francisco, so a farm supply store looks a little odd. The
store closed (just this one out of the chain) and Orchard Supply Hardware
moved in. Evidently they didn't like the building, and it was razed, and
a new one put in, much larger. There was no longer room for my OD and
some other small places. There was room elsewhere in the shopping center,
though. At one visit he told me that he was moving around the corner. A
year later, at my next visit, I was amazed to see that his new office was
absolutely identical, down to the colors and wood trim. When I went into
the exam room, I was even more astonished to see the same odd layout,
which I had always thought was due to circumstances beyond his control. I
commented on how it seemed that his office looked identical to the old
one. He said that it was no coincidence, that a condition of his move was
that they reproduce his office exactly.

[Dan Abel]

Post by David Robins, MD
20 feet (about 6 meters) is not at the limit of measurement. The
accommodation for that distance is 1/6 meters, which is 1/6 of a diopter.
So, if you refract perfectly at 20 feet, one should add 1/6 of a diopter
more minus to go out to infinity. Lenes typically come in 0.25D increments,
and can be ordered in 0.12D increments (1/8 of a diopter.) ANSI lens
manufacturing standards are about +/1 0.13D iup to 6.50 diopters.

I stand corrected. I wear contacts and OTC reading glasses, and they
don't come in strengths that reflect the difference between 20' and
infinity. Furthermore, back when I was very myopic, the contacts I needed
only came in increments of .5D, so that's where I'm coming from.

[David Robins, MD]

True, you can't order contacts 1/6 D more power. I think many refractionists
will add 0.25D if they really want sharper distance vision - at least I used
to when I was refracting. Contacts can be ordered in 0.25D increments, as I
recall. Certainly spectacles can be, which can be specified dow to 1/8 D.

This is sometimes also done using the red-green test, using the
green-sharper image as the endpoint, rather than the equal red-green which
would be in focus at 20 feet.



On 7/2/04 10:22 AM, in article

Post by Dan Abel

Post by David Robins, MD
20 feet (about 6 meters) is not at the limit of measurement. The
accommodation for that distance is 1/6 meters, which is 1/6 of a diopter.
So, if you refract perfectly at 20 feet, one should add 1/6 of a diopter
more minus to go out to infinity. Lenes typically come in 0.25D increments,
and can be ordered in 0.12D increments (1/8 of a diopter.) ANSI lens
manufacturing standards are about +/1 0.13D iup to 6.50 diopters.

I stand corrected. I wear contacts and OTC reading glasses, and they
don't come in strengths that reflect the difference between 20' and
infinity. Furthermore, back when I was very myopic, the contacts I needed
only came in increments of .5D, so that's where I'm coming from.

[andrew Judd]

Post by David Robins, MD
20 feet (about 6 meters) is not at the limit of measurement. The
accommodation for that distance is 1/6 meters, which is 1/6 of a diopter.

Hmmmmmm. Is this true? As a fact? or is it a guess?

I knew there was a rule of thumb that one diopter of myopia was
equivalent to one meter of clarity, and you could use this
relationship to then have a pretty good idea of self testing myopia by
measuring the exact point at which blur was not present.

So if you can see perfectly clear at 25 CM you were -4D etc

So 2M = .5D half

4M = .25D Quarter

8M = .125D Eighth

So it appears that 1/6 for 6M is following exactly this pretty
reliable guesstimation system.

[David Robins, MD]

That is to optical fact. Focusing at 6 meters 1/6 diopter of accommodation.
That is how I arrived to state it: it is the inverse of the distance in
meters.

On 7/4/04 4:37 AM, in article

Post by andrew Judd

Post by David Robins, MD
20 feet (about 6 meters) is not at the limit of measurement. The
accommodation for that distance is 1/6 meters, which is 1/6 of a diopter.

Hmmmmmm. Is this true? As a fact? or is it a guess?
I knew there was a rule of thumb that one diopter of myopia was
equivalent to one meter of clarity, and you could use this
relationship to then have a pretty good idea of self testing myopia by
measuring the exact point at which blur was not present.
So if you can see perfectly clear at 25 CM you were -4D etc
So 2M = .5D half
4M = .25D Quarter
8M = .125D Eighth
So it appears that 1/6 for 6M is following exactly this pretty
reliable guesstimation system.

[andrew Judd]

Post by David Robins, MD
That is to optical fact. Focusing at 6 meters 1/6 diopter of accommodation.
That is how I arrived to state it: it is the inverse of the distance in
meters.

Yes. It now seems obvious that my guesstimation system is actually
based on science rather than a pure guess!

But it does seem strange that 6/6 is actually underprescribed, albeit
by a tiny amount. Naturally that is going to possibly create
difficulties for those people who are then further underprescribed.

Interesting also that 20 feet provides a noticably exo deviation of
the eyes compared to infinity. I recall that at 20 feet it is about 6
inches difference - ie a small mark on a window at 20 feet, apparently
moves about 6 inches apart when the eyes look at infinity 'between'
the now doubled mark.

These differences might seem trivial. But it occurs to me that if a
person has a 'cant be bothered' attitude or impatient attitude towards
seeing they may not allow their total eye mind system the time
required to make these small changes and instead get annoyed or
impatient for better vision. Important when you consider that myopes
have lags in the accommodation system compared to normal sighted
people, and commonly have various exo and eso errors in directing
their eyes.