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Science Forum Index » Medicine - Vision Forum » Can difference in vision be explained?
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| katy via MedKB.com |
 Posted: Fri Dec 29, 2006 3:22 am |
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Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
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Message posted via MedKB.com
http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 |
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| Dr. Leukoma |
| Posted: Fri Dec 29, 2006 9:00 am |
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katy via MedKB.com wrote:
Quote: Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
OK, here is something for starters.
There are optical limits and neurological limits. There are many
variations in people's optical systems. Currently, the best way to
measure this is with something called aberrometry, which is quite a bit
more sophisticated than the methods used in the traditional eye exam.
The pupil also plays an important role in fine-tuning the optical
system. The pupil limits the influence that optical perfections have
in distorting vision. Saying that two people have the same
prescription is like saying that two people look the same because they
have the same height or the same weight. It ignores the facial
features.
DrG |
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| otisbrown@pa.net |
| Posted: Fri Dec 29, 2006 11:34 am |
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Dear Katy,
In fact there are two DIFFERENT methods of judging
a person's refractive STATE.
1. Use a Snellen and a trial lens kit. (The so-called
"subjective" measurement). In this method you
have the person read the snellen at say 20/50. You
then use stronger minus lenses until the 20/20
line is cleared, with say a -3/4 diopter lens.)
2. Measure with a retinoscope/cycloplegic (drops),
and determine a person's refractive STATE.
These two methods often do not agree -- and
can be profoundly different.
In fact it is possible to have 20/20 and a reafractive
STATE (cycloplegic) of -3/4 diopters.
This my explain some of the difference between
your refractive measurement and your current
visual acuity.
It is also true as suggested by the ODs on
sci.med.vision, that as you get older your
refractive STATE will change in a positive
direction, and your Snellen will clear.
That might also explain why you can
get along with no minus lens.
Obviously, opinions vary concerning your
distant visual acuity.
Best,
Otis
katy via MedKB.com wrote:
Quote: Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
--
Message posted via MedKB.com
http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 |
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| otisbrown@pa.net |
| Posted: Fri Dec 29, 2006 11:41 am |
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Dear Katy,
I would also add, that sometimes the difference between
Snellen and Retinoscope/cycloplegic can be profound.
For instance, one 2 year-old had a Snellen of
20/60 -- which is functional for a child of that age -- and
a retinoscope/cycloplegic of -11 diopters.
So what do you do? Tell the mother that 20/60 is
OK for now, or "prescribe" based on the retinoscope/cycloplegic.
For the case in question, the child is now wearing a -10 dioper
lens all the time.
Obviously there is a majority and second opinion on this
subject.
Just one man's opinion.
Best,
Otis
otisbrown@pa.net wrote:
Quote: Dear Katy,
In fact there are two DIFFERENT methods of judging
a person's refractive STATE.
1. Use a Snellen and a trial lens kit. (The so-called
"subjective" measurement). In this method you
have the person read the snellen at say 20/50. You
then use stronger minus lenses until the 20/20
line is cleared, with say a -3/4 diopter lens.)
2. Measure with a retinoscope/cycloplegic (drops),
and determine a person's refractive STATE.
These two methods often do not agree -- and
can be profoundly different.
In fact it is possible to have 20/20 and a reafractive
STATE (cycloplegic) of -3/4 diopters.
This my explain some of the difference between
your refractive measurement and your current
visual acuity.
It is also true as suggested by the ODs on
sci.med.vision, that as you get older your
refractive STATE will change in a positive
direction, and your Snellen will clear.
That might also explain why you can
get along with no minus lens.
Obviously, opinions vary concerning your
distant visual acuity.
Best,
Otis
katy via MedKB.com wrote:
Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
--
Message posted via MedKB.com
http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 |
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| Dan Abel |
| Posted: Fri Dec 29, 2006 12:49 pm |
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In article <6b7944188fdcb@uwe>, "katy via MedKB.com" <u12444@uwe>
wrote:
Quote: Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
There are a lot of factors, for instance cataract. -3 doesn't tell how
well someone sees, it is just the correction that gives best vision for
that individual.
Don't pay any attention to Otis. |
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| otisbrown@pa.net |
| Posted: Fri Dec 29, 2006 2:21 pm |
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Statement of a "concerned" Parent of 3 year-old:
I have posted several threads about my 3 year old on this site(-10.5
but sees
20/60 without glasses). My question is that a year ago when I brought
my
daughter in to the pediatric opthalmologist he originally had her
perscription at -12.50 each eye. He gave her a lighter perscription to
start
out with though(-10.00). About 3 months later she was -11.50 each eye.
About
3 months later he had her at about -11.00 each eye. Then about 3 months
later
he had her at -10.50 each eye. Just recently when she went back she was
-10.
25 in one eye and -10.00 in the other. The astigmatism has been about
the
same for all perscriptions. I was not at the last visit and would have
asked
him why its going down. I am not posting this in ANYWAY to start any
controversy. We have not been doing ANY sort of vision therapy or
trying to
improve her vision in anyway, shape, or form. I was wondering if it
made any
difference that she has lenticular myopia or if because she is young
maybe
her lens could be changing still.
Signed,
A concerned parent
otisbrown@pa.net wrote:
Quote: Dear Katy,
I would also add, that sometimes the difference between
Snellen and Retinoscope/cycloplegic can be profound.
For instance, one 2 year-old had a Snellen of
20/60 -- which is functional for a child of that age -- and
a retinoscope/cycloplegic of -11 diopters.
So what do you do? Tell the mother that 20/60 is
OK for now, or "prescribe" based on the retinoscope/cycloplegic.
For the case in question, the child is now wearing a -10 dioper
lens all the time.
Obviously there is a majority and second opinion on this
subject.
Just one man's opinion.
Best,
Otis
otisbrown@pa.net wrote:
Dear Katy,
In fact there are two DIFFERENT methods of judging
a person's refractive STATE.
1. Use a Snellen and a trial lens kit. (The so-called
"subjective" measurement). In this method you
have the person read the snellen at say 20/50. You
then use stronger minus lenses until the 20/20
line is cleared, with say a -3/4 diopter lens.)
2. Measure with a retinoscope/cycloplegic (drops),
and determine a person's refractive STATE.
These two methods often do not agree -- and
can be profoundly different.
In fact it is possible to have 20/20 and a reafractive
STATE (cycloplegic) of -3/4 diopters.
This my explain some of the difference between
your refractive measurement and your current
visual acuity.
It is also true as suggested by the ODs on
sci.med.vision, that as you get older your
refractive STATE will change in a positive
direction, and your Snellen will clear.
That might also explain why you can
get along with no minus lens.
Obviously, opinions vary concerning your
distant visual acuity.
Best,
Otis
katy via MedKB.com wrote:
Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
--
Message posted via MedKB.com
http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1 |
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| Mike Tyner |
| Posted: Fri Dec 29, 2006 3:02 pm |
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<otisbrown@pa.net> wrote
Quote: In fact there are two DIFFERENT methods of judging
a person's refractive STATE.
Yes but your two "methods" don't really address her question, do they?
She is asking why you can't predict VA from refraction.
Quote: then use stronger minus lenses until the 20/20
line is cleared, with say a -3/4 diopter lens.)
2. Measure with a retinoscope/cycloplegic (drops),
and determine a person's refractive STATE.
Is a cycloplegic necessary to do retinoscopy? Can't I use lenses with
cycloplegic?
Quote: These two methods often do not agree -- and
can be profoundly different.
Perhaps because they don't measure the same thing.
Quote: In fact it is possible to have 20/20 and a reafractive
STATE (cycloplegic) of -3/4 diopters.
And it's possible you don't know what you're talking about.
Quote: This my explain some of the difference between
your refractive measurement and your current
visual acuity.
Not a whit.
Pick either of your "methods," then find two people who measure the same.
Why does one see better than the other without glasses?
Hint: Answers include pupil size, higher-order aberrations, accommodative
spasm and erroneous refraction, plus neurology, and psychology, and there's
also the possibility that measured VA might NOT vary quite as much as she
thinks it does.
Quote: Obviously, opinions vary concerning your
distant visual acuity.
She wasn't asking for opinions. She was asking why subjective acuity varies
from one person to another your refractive STATE is the same.
-MT
Quote: katy via MedKB.com wrote:
Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example,
I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
--
Message posted via MedKB.com
http://www.medkb.com/Uwe/Forums.aspx/vision/200612/1
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| Neil Brooks |
| Posted: Fri Dec 29, 2006 4:05 pm |
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Salmon Egg wrote:
Quote: On 12/29/06 7:41 AM, in article
1167406898.484719.25500@i12g2000cwa.googlegroups.com, "otisbrown@pa.net"
otisbrown@pa.net> wrote:
For instance, one 2 year-old had a Snellen of
20/60 -- which is functional for a child of that age -- and
a retinoscope/cycloplegic of -11 diopters.
How can you even rely upon a 2 year-old to read a Snellen chart correctly?
His mind is made up. Please do not try to confuse him with logic or
facts. His head could explode.... |
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| Neil Brooks |
| Posted: Fri Dec 29, 2006 4:07 pm |
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otisbrown@pa.net wrote:
Quote: I would also add, that sometimes the difference between
Snellen and Retinoscope/cycloplegic can be profound.
NEARLY as profound as the difference between Otis's statements and
actual fact.
Nearly. |
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| Dr. Leukoma |
| Posted: Fri Dec 29, 2006 4:10 pm |
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Neil Brooks wrote:
Quote: His mind is made up. Please do not try to confuse him with logic or
facts. His head could explode....
OK, I am finally willing to admit that the load of bricks might be
missing a few....
DrG |
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| William Stacy |
| Posted: Fri Dec 29, 2006 6:31 pm |
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All other things being equal, pupil size is far and away the biggest
determiner of unaided acuity between two myopes. Just like a camera, if
you set the focus at a particular distance, changing the f-stop will
profoundly affect the photo. That's why cheap cameras with tiny
apertures don't really need any focusing mechanism, while expensive ones
with large lenses and variable f-stops need such fine focusing. In the
extreme example, take the -3 patient and have them look through a
pinhole. They will be close to 20/20 without any lens at all. Take the
pinhole away and they'd be lucky to get 20/200.
w.stacy, o.d.
katy via MedKB.com wrote:
Quote: Is there a scientific explanation for why two people with the same
prescription appear to have different levels of vision? For example, I've
read somewhere that two people who are -3 won't necessarily see the same
without glasses, one might see better/worse than the other. What factors
influence this?
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| William Stacy |
| Posted: Fri Dec 29, 2006 6:58 pm |
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Oh, and this effect can be (and often is) utilized to distort acuity
measurements. Allow the patient to squint (squeeze the lids together),
and the reduction in the "aperture" will allow him to see smaller
letters. This is why you see myopes squinting a lot when their glasses
are too weak or when they don't have them. Similarly, flip on the
overhead lighting and the pupils constrict, giving better acuity. Turn
all lights off, and they do worse. The other big difference is
psychological. Some people are far more profoundly bothered by blur
than others. Some will feel completely happy in a -3 fog while others
will feel like a fish out of water.
w.stacy, o.d.
William Stacy wrote:
Quote: Take the pinhole away and they'd be lucky to get 20/200.
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| Scott Seidman |
| Posted: Fri Dec 29, 2006 7:04 pm |
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William Stacy <wstacy@obase.net> wrote in news:b4hlh.41115$wc5.587
@newssvr25.news.prodigy.net:
Quote: This is why you see myopes squinting a lot when their glasses
are too weak or when they don't have them
But doesn't the squinting cause the eyeball to shrink and cure the myopia??
Sorry Bill, long week, and no self control left
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Scott
Reverse name to reply |
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| VicTek |
| Posted: Fri Dec 29, 2006 7:25 pm |
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Quote: are too weak or when they don't have them. Similarly, flip on the
overhead lighting and the pupils constrict, giving better acuity. Turn
all lights off, and they do worse.
I have noticed for a long time that my distance vision, which is corrected
with glasses, is degraded when driving at night. Is the above the
explanation for that? If so how can one compensate for the problem -
different prescriptions for day and night use? |
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| Mike Tyner |
| Posted: Fri Dec 29, 2006 8:28 pm |
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"VicTek" <abc@xyz.com> wrote
Quote: I have noticed for a long time that my distance vision, which is corrected
with glasses, is degraded when driving at night. Is the above the
explanation for that? If so how can one compensate for the problem -
different prescriptions for day and night use?
You could, but a precise prescription should work for both.
In your circumstances, it's often that the glasses are a little off but it's
only evident when the pupils are large and depth-of-field is short.
-MT |
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