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Is it me, my eyes, or what? (1 Viewer)
- Thread starter giosblue
- Start date
It is difficult to find actual Snellen tests of native Americans, Africans etc.
But Darwin reported that native south American sailors could see distant objects much better than any of his European sailors. It was only with a telescope that they could confirm what they thought could not be seen with unaided eyes.
Apparently native south Americans are claimed to share ancient DNA with Aboriginal Australians.
It was reported that Aboriginal Australians could see one line further down a test chart with one eye than European Australians, but two lines further down with binocular vision. It was suggested that their visual cortex processed information differently.
As to why more 20/8 vision isn't found, I think it is because opticians only look for 20/20 or 20/15 vision. The Snellen charts are truncated, so maybe even 20/10 vision isn't recorded.
Also the opticians say 20 feet is 'infinity' when it isn't
In addition, young people probably have the best eyes and don't need to see an optician, so their vision is never tested till they are perhaps 40 years old.
School eye tests are very basic. My friend who had serious eye damage in one eye was able to bluff his way through the test.
I wonder if anyone has got a friend with better eyesight to take an eye test or indeed a car driving eye test.
It is clear that myopia is increasing in modern society because of long periods of watching screens and maybe incorrect diet.
But Darwin reported that native south American sailors could see distant objects much better than any of his European sailors. It was only with a telescope that they could confirm what they thought could not be seen with unaided eyes.
Apparently native south Americans are claimed to share ancient DNA with Aboriginal Australians.
It was reported that Aboriginal Australians could see one line further down a test chart with one eye than European Australians, but two lines further down with binocular vision. It was suggested that their visual cortex processed information differently.
As to why more 20/8 vision isn't found, I think it is because opticians only look for 20/20 or 20/15 vision. The Snellen charts are truncated, so maybe even 20/10 vision isn't recorded.
Also the opticians say 20 feet is 'infinity' when it isn't
In addition, young people probably have the best eyes and don't need to see an optician, so their vision is never tested till they are perhaps 40 years old.
School eye tests are very basic. My friend who had serious eye damage in one eye was able to bluff his way through the test.
I wonder if anyone has got a friend with better eyesight to take an eye test or indeed a car driving eye test.
It is clear that myopia is increasing in modern society because of long periods of watching screens and maybe incorrect diet.
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mooreorless
Well-known member
"As to why more 20/8 vision isn't found, I think it is because opticians only look for 20/20 or 20/15 vision. The Snellen charts are truncated, so maybe even 20/10 vision isn't recorded.
Also the opticians say 20 feet is 'infinity' when it isn't"
"It is clear that myopia is increasing in modern society because of long periods of watching screens and maybe incorrect diet."
I would have to agree with both of these 100%
Also the opticians say 20 feet is 'infinity' when it isn't"
"It is clear that myopia is increasing in modern society because of long periods of watching screens and maybe incorrect diet."
I would have to agree with both of these 100%
looksharp65
Well-known member
Can't speak on behalf of others, but even when someone fails two of five letters, I usually show the next line.
Rarely do I meet people with 2.0 (20/10) acuity, but many can identify four of five letters on the 1.5 line (roughly 20/13). Cropping one line or occasionally one single letter makes readability better, not worse.
In my exam room, I have an old/ish Topcon ACP-8 projector, whereas my colleague prefers the new LCD screen.
The projected image is somewhat lacking in contrast, although I sometimes turn off the ambient light to see if the acuity increases. I must have subdued ambient light to reach meaningful VA, but the final VA record can often be a bit lower than in rooms with a LCD screen, or for that matter with people who visited the ophtalmologist with their high contrast printed test screens.
Re. 20 feet, let's first realise that spectacle lenses are ordered in 0.25 diopter steps. If someone is perfectly ametropic, a +0.25 makes the P.R. (punctum remotum) move from infinity to +4 meters. Consequently, there is no built-in lens in the phoroptor that puts the P.R. at 20 ft, leaving 0.17 D uncorrected.
The projector has one advantage over printed screens, a slightly shorter projection distance means a smaller image, so the VA numbers won't become inflated. Theoretically, the chart could be at 2 meters if the refraction is corrected with -0.50 D, which technically could also be built-in so the interface shows the optometrist the correct distance refraction, rather than the actual powers of the prevalent lenses in the phoroptor.
The only time a too short a room can cause real trouble is when checking the uncorrected VA and the P.R. due to myopia coincides with the room's length. Like, a 4 meter room would suggest perfect vision for a -0.25 myope.
If there are such rooms, I presume the professional takes measures to avoid getting skewed results.
Recognising such low confidence for my colleagues makes me sad, regardless of whether it's warranted or just emanating from laymen's assumptions.
//L
Rarely do I meet people with 2.0 (20/10) acuity, but many can identify four of five letters on the 1.5 line (roughly 20/13). Cropping one line or occasionally one single letter makes readability better, not worse.
In my exam room, I have an old/ish Topcon ACP-8 projector, whereas my colleague prefers the new LCD screen.
The projected image is somewhat lacking in contrast, although I sometimes turn off the ambient light to see if the acuity increases. I must have subdued ambient light to reach meaningful VA, but the final VA record can often be a bit lower than in rooms with a LCD screen, or for that matter with people who visited the ophtalmologist with their high contrast printed test screens.
Re. 20 feet, let's first realise that spectacle lenses are ordered in 0.25 diopter steps. If someone is perfectly ametropic, a +0.25 makes the P.R. (punctum remotum) move from infinity to +4 meters. Consequently, there is no built-in lens in the phoroptor that puts the P.R. at 20 ft, leaving 0.17 D uncorrected.
The projector has one advantage over printed screens, a slightly shorter projection distance means a smaller image, so the VA numbers won't become inflated. Theoretically, the chart could be at 2 meters if the refraction is corrected with -0.50 D, which technically could also be built-in so the interface shows the optometrist the correct distance refraction, rather than the actual powers of the prevalent lenses in the phoroptor.
The only time a too short a room can cause real trouble is when checking the uncorrected VA and the P.R. due to myopia coincides with the room's length. Like, a 4 meter room would suggest perfect vision for a -0.25 myope.
If there are such rooms, I presume the professional takes measures to avoid getting skewed results.
Recognising such low confidence for my colleagues makes me sad, regardless of whether it's warranted or just emanating from laymen's assumptions.
//L
Thanks Lars.
The paper on the Visual acuity of 163 Navy pilots, 145 of whom flew F14 Tomcats, with an average of 20/8 high contrast Snellen VA points out that only 20/20 vision was measured by their opticians and it is necessary to have an optimal correction.
Morris and Hamilton, referenced in post 73.
This was in 1986, so maybe things have improved.
My opthalomogist has definitely written my prescription to 0.125 dioptres and the laboratory head follows his prescription as well as he can. The astigmatism angle is also written to 2.5 degrees.
He said that in about 30 years, only about three people, including me, had used his 1/8 dioptre lens.
I use a wall about 37 metres away to get nearer to 'infinity' for my astro glasses.
He tells the laboratory head that I am a crazy astronomer. He is right.
I don't think that opticians are doing anything wrong with using 20ft as infinity or not getting the best correction.
It takes 75 to 90 minutes for my opthalmologist to get my prescription as best as we can get it.
Because of age, I don't pay for annual eye tests, but he definitely would lose money on my tests.
However, I pay more for any glasses than I would at somewhere like Specsavers, and am more than willing to do this.
Why spend a lot on a binocular and little on ones own eyes?
To me this is daft, although my Scottish friend spends next to nothing on glasses but a lot on other optics.
P.S.
Lars,
Re. post 68.
Do you have access to the actual test results on Veronica Seider's visual acuity when she was about 20 years old?
The paper on the Visual acuity of 163 Navy pilots, 145 of whom flew F14 Tomcats, with an average of 20/8 high contrast Snellen VA points out that only 20/20 vision was measured by their opticians and it is necessary to have an optimal correction.
Morris and Hamilton, referenced in post 73.
This was in 1986, so maybe things have improved.
My opthalomogist has definitely written my prescription to 0.125 dioptres and the laboratory head follows his prescription as well as he can. The astigmatism angle is also written to 2.5 degrees.
He said that in about 30 years, only about three people, including me, had used his 1/8 dioptre lens.
I use a wall about 37 metres away to get nearer to 'infinity' for my astro glasses.
He tells the laboratory head that I am a crazy astronomer. He is right.
I don't think that opticians are doing anything wrong with using 20ft as infinity or not getting the best correction.
It takes 75 to 90 minutes for my opthalmologist to get my prescription as best as we can get it.
Because of age, I don't pay for annual eye tests, but he definitely would lose money on my tests.
However, I pay more for any glasses than I would at somewhere like Specsavers, and am more than willing to do this.
Why spend a lot on a binocular and little on ones own eyes?
To me this is daft, although my Scottish friend spends next to nothing on glasses but a lot on other optics.
P.S.
Lars,
Re. post 68.
Do you have access to the actual test results on Veronica Seider's visual acuity when she was about 20 years old?
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People probably don't visit a doctor or optician unless there is something wrong.
I don't think that I visited an optician until I was about 40, because I didn't need to.
It was only when I had trouble reading that I went to the optician and got reading glasses.
It does not surprise me that opticians don't see people with 20/8 sight. These people are probably young and have no need at all to see an optician.
I know that private opticians in the U.K. have suffered greatly from the big low cost groups.
Firstly, most people are not that much concerned with having their eyes minutely tested, and they expect glasses for little cost.
My friend was in St Petersburg and got prescription glasses for about £3.
I don't think that I visited an optician until I was about 40, because I didn't need to.
It was only when I had trouble reading that I went to the optician and got reading glasses.
It does not surprise me that opticians don't see people with 20/8 sight. These people are probably young and have no need at all to see an optician.
I know that private opticians in the U.K. have suffered greatly from the big low cost groups.
Firstly, most people are not that much concerned with having their eyes minutely tested, and they expect glasses for little cost.
My friend was in St Petersburg and got prescription glasses for about £3.
looksharp65
Well-known member
People with extremely good eyesight may, or may not be aware of their ability. Because it's the majority's typical performance that usually sets the bar in their daily habitat, which will not disfavour those with high performance, only those with decidedly reduced performance. If a printed newspaper scales down the letters to half their previous size, there will be a few individuals who experience no problem with this, despite having hyperopia and/or presbyopia.
The reason for this reads in a thumb rule: If there's absolute ametropia (or distance correction is perfect to equal ametropia), a +1.0 D lens will dimidiate the VA (pupil's size effect on depth of field aside).
So a person with a 20/10 VA and a -1.0 D myopia may still achieve 20/20 vision and, due to not having met higher requirements actually be unaware of their myopia.
At the other end of the scale, the problem is a lot more pronounced where elderly people, used to being able to read small print at a fairly long reading distance, can't accept to now read large text only, can't accept to use magnifying glasses and can't accept reading glasses with high power to reduce the reading distance.
---
I have encountered "optical wisdom" saying that after having reached a preliminary refraction, +1.0 should be added and slowly be reduced until 1.0 (20/20) is reached and never go further towards minus and/or better VA. Obviously, this is worse than silly. In my opinion, it's misconduct.
I have also met lecturers who dissuade from trying to always obtain the best VA possible, with the twisted logic that nobody can improve their vision at every visit into eternity. While this is true, the idea inspired from this is downright silly.
Other "optical wisdom" tells to avoid correcting astigmatism, if necessary stick with 90/180 degrees axis, if oblique axis, going for "mirrored" axis and so on. Doing this by routine will keep the average VA down, may mean some possible adaptation problems can be avoided, but IMO at the expense of others who will complain about lacking sharpness.
However, from my experience, a very high VA can be a curse when spectacles are introduced because the wearer will, by boosting the central visual acuity, also be able to detect optical shortcomings a lot easier.
Explaining to someone who can see absolutely minuscule detail in the center of the reading field why sharpness decreases so rapidly towards the periphery is no easy task. It is the comparison between the very good and the outlandish sharpness that makes the VG look mediocre. The same goes for the depth of field, while more rarely encountered than the lateral type.
Spectacles are a poor compromise in so many ways - for those with low vision because they can only provide magnification at short distance, for those with grandiose vision because they can detect the spectacle's shortcomings, even for some of those with ordinary vision because of distortions unavoidable with any lens power and so much more.
But it has to be stated that it's not usually the prescription policy (best possible VA vs. "wisdom"), neither lacking equipment or testing conditions, neither lacking skills that causes complaints at spectacles.
I mentioned a few above, others are anisometropia, aniseikonia, a need for correcting small phoria (esp. vertical or oblique) when both eyes begin to see sharp, it's poor adjustment and/or wearing habits, failure to clean the glasses properly, and sometimes downright lack of will to accept the spectacles. A small percentage of customers also have some or another type of obsession around their spectacle lenses.
But you know, they're only pieces of plastics in a rack on your nose to help you see better
//L
The reason for this reads in a thumb rule: If there's absolute ametropia (or distance correction is perfect to equal ametropia), a +1.0 D lens will dimidiate the VA (pupil's size effect on depth of field aside).
So a person with a 20/10 VA and a -1.0 D myopia may still achieve 20/20 vision and, due to not having met higher requirements actually be unaware of their myopia.
At the other end of the scale, the problem is a lot more pronounced where elderly people, used to being able to read small print at a fairly long reading distance, can't accept to now read large text only, can't accept to use magnifying glasses and can't accept reading glasses with high power to reduce the reading distance.
---
I have encountered "optical wisdom" saying that after having reached a preliminary refraction, +1.0 should be added and slowly be reduced until 1.0 (20/20) is reached and never go further towards minus and/or better VA. Obviously, this is worse than silly. In my opinion, it's misconduct.
I have also met lecturers who dissuade from trying to always obtain the best VA possible, with the twisted logic that nobody can improve their vision at every visit into eternity. While this is true, the idea inspired from this is downright silly.
Other "optical wisdom" tells to avoid correcting astigmatism, if necessary stick with 90/180 degrees axis, if oblique axis, going for "mirrored" axis and so on. Doing this by routine will keep the average VA down, may mean some possible adaptation problems can be avoided, but IMO at the expense of others who will complain about lacking sharpness.
However, from my experience, a very high VA can be a curse when spectacles are introduced because the wearer will, by boosting the central visual acuity, also be able to detect optical shortcomings a lot easier.
Explaining to someone who can see absolutely minuscule detail in the center of the reading field why sharpness decreases so rapidly towards the periphery is no easy task. It is the comparison between the very good and the outlandish sharpness that makes the VG look mediocre. The same goes for the depth of field, while more rarely encountered than the lateral type.
Spectacles are a poor compromise in so many ways - for those with low vision because they can only provide magnification at short distance, for those with grandiose vision because they can detect the spectacle's shortcomings, even for some of those with ordinary vision because of distortions unavoidable with any lens power and so much more.
But it has to be stated that it's not usually the prescription policy (best possible VA vs. "wisdom"), neither lacking equipment or testing conditions, neither lacking skills that causes complaints at spectacles.
I mentioned a few above, others are anisometropia, aniseikonia, a need for correcting small phoria (esp. vertical or oblique) when both eyes begin to see sharp, it's poor adjustment and/or wearing habits, failure to clean the glasses properly, and sometimes downright lack of will to accept the spectacles. A small percentage of customers also have some or another type of obsession around their spectacle lenses.
But you know, they're only pieces of plastics in a rack on your nose to help you see better
//L
Sorry I do not quite understand. Is it very unusual to have high acuity and fairly advanced myopia? When such people visit an optometrist/ophthalmologist for glasses/contacts to correct their myopia will not their acuity also be tested (to whatever precision)?
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Hi adhoc,
Rev Dawes had high acuity and was very short sighted, if my memory serves me correctly, such that he couldn't recognise people without his glasses.
However, a person with severe myopia will probably be very thankful if glasses give him or her 20/15 vision.
I doubt that there are many who want to go to the levels that I do with my eye tests.
I recently read that large numbers of army reservists had terrible eyes. They hadn't seen an optician for years, and generally had vision that did not meet basic requirements.
Interestingly, an old paper suggested that diagonal VA was 10% to 20% less than horizontal or vertical VA using standard USAF test charts.
I thought it strange that pilots only needed 20/200 vision without glasses and 20/20 with.
The RAF requires pilots to have 20/20 vision or better, with and without glasses. This is a stricter standard than for many other pilots.
I just looked at how much the NHS pays for free over age 60 patient eye tests, and it is not enough to fund a 90 minute eye test.
The opthalmologist pays rent, rates, running costs and has a full time assistant.
A family friend opthalmologist gave up working because of stress trying to compete with the big chains of opticians.
This is similar to the British high street being decimated by the likes of Amazon, who pay little tax here and have in my opinion, almost zero regard to human beings in general.
Rev Dawes had high acuity and was very short sighted, if my memory serves me correctly, such that he couldn't recognise people without his glasses.
However, a person with severe myopia will probably be very thankful if glasses give him or her 20/15 vision.
I doubt that there are many who want to go to the levels that I do with my eye tests.
I recently read that large numbers of army reservists had terrible eyes. They hadn't seen an optician for years, and generally had vision that did not meet basic requirements.
Interestingly, an old paper suggested that diagonal VA was 10% to 20% less than horizontal or vertical VA using standard USAF test charts.
I thought it strange that pilots only needed 20/200 vision without glasses and 20/20 with.
The RAF requires pilots to have 20/20 vision or better, with and without glasses. This is a stricter standard than for many other pilots.
I just looked at how much the NHS pays for free over age 60 patient eye tests, and it is not enough to fund a 90 minute eye test.
The opthalmologist pays rent, rates, running costs and has a full time assistant.
A family friend opthalmologist gave up working because of stress trying to compete with the big chains of opticians.
This is similar to the British high street being decimated by the likes of Amazon, who pay little tax here and have in my opinion, almost zero regard to human beings in general.
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