It's very simple peterg. I found the method myself intuitively. Later i found it was also correct mathematically.
I am going to re-write some of the things i said before. Follow me carefully.
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According to the standard chart (not in absolute terms).A person without glasses and with myopia of :
0 = clear image at infinity (nobody has this in absolute terms - nobody can see infinitely small details at infinite distances, at least not in the material universe)
0.25 = clear image at 4 meters.
0.5 = clear image at 2 meters.
0.75 = clear image at 1.3 meters.
1 = clear image at 1 meter.
1.25 = clear image at 80 centimeters
1.5 = clear image at 66 centimeters
1.75 = clear image at 57 centimeters
2 = clear image at 50 centimeters
2.25 = clear image at 44 centimeters
2.5 = clear image at 40 centimeters
2.75 = clear image at 36.36 centimeters
3 = clear image at 33 centimeters
3.25 = clear image at 30.7 centimeters
3.5 = clear image at 28.57 centimeters
3.75 = clear image at 26.66 centimeters
4 = clear image at 25 centimeters
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You do the above math by dividing 1 with the diopter.
Example : A person with -4.25 myopia can see clear at 1 / 4.25 = 0.235 meaning 23.5 centimeters.
Now the Snellen chart shows the same thing. (not in absolute terms)
20/40
(or 6m/12m) literally means 20 divided by 40
(or 6 divided 12) which equals 0.5 meaning clear image at 50 centimeters (1 divided by 2 diopters = 50 centimeters) -> So you have -2 myopia if you read the 20/40.
20/30 literally means 20 divided by 30 which equals 0.66 meaning clear image at 66 centimeters (1 divided by 1.5 diopters = 66 centimeters) -> So you have -1.5 myopia if you read the 20/30.
20/25 literally means 20 divided by 25 which equals 0.8 meaning clear image at 80 centimeters (1 divided by 1.25 diopters = 80 centimeters) -> So you have -1.25 myopia if you read the 20/25.
20/20 simply means that you see the letters at 6 meters correctly, at that particular size. It also means that if you see the text easily you have
in absolute terms a value closer to 0 diopters. If you see the text blurry but still can read some of the letters you have
in absolute terms a maximum of -1.
Absolute terms means beyond the chart.
To clarify things please read this short article :
http://www.visionhelp.com/vh_about_13.htmlBeyond 20/20 the chart shows the true power of the eye in absolute terms, since with 20/20 you have 0 diopters to a doctor's perspective but in reality the truth is you can never have 0.
In absolute terms :20/15 = 1.3 meters of perfect image (0.75 myopia)
20/10 = 2 meters of perfect image (0.5 myopia)
20/8 = 2.5 meters of perfect image (0.4 myopia)
The hawk has 20/2 = 10 meters of perfect image (0.1 myopia)
http://en.wikipedia.org/wiki/Visual_acuity"If the optics of the eye were otherwise perfect, theoretically, acuity would be limited by pupil diffraction, which would be a diffraction-limited acuity of 0.4 minutes of arc (minarc) or 20/8 acuity. The smallest cone cells in the fovea have sizes corresponding to 0.4 minarc of the visual field, which also places a lower limit on acuity. The optimal acuity of 0.4 minarc or 20/8 can be demonstrated using a laser interferometer that bypasses any defects in the eye's optics and projects a pattern of dark and light bands directly on the retina. Laser interferometers are now used routinely in patients with optical problems, such as cataracts, to assess the health of the retina before subjecting them to surgery."
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20/5 has never been recorded in human vision.
20/8 has been, but is very rare. It is the best recorded human vision
possible.
20/10 isn't even all that common. Is less common, but not all that rare.
20/15 is already better visual acuity than most will ever enjoy. Is even better and is quite common.
20/20 is the standard set as good, normal vision.
Hawks, have been measured at 20/2....hence the phrase, eyes like a hawk. A hawk has 20/2 vision , which is the best in the animal kingdom. 20/10 means you can see at 20 feet what a person with good normal vision can see only when they are 10 feet away.
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If you try my values in the calculator you will i see that my math is flawless. For example my edge of blue is now 50 centimeters. If i give a prescription of -2, the calculator will say it is a perfect prescription, according to the 20/20 system of course.
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When you naturally reach 20/8 there is no Over-prescription that could make your eyes work like a hawk, because of the human eye limitation. You need to change your eyes with robotic ones.
