Just about everyone with defective vision desires 20/20 for their distance vision. What is 20/20 vision anyway? it’s a benchmark measure of what you can see from 20 feet away.
The typical way to measure your visual acuity for distance has been using the Snellen Eye Chart. The natural vision students we teach learn to read the Snellen Chart to improve their vision as well as to measure their improvement in distance vision.
Visual Acuity Test using Snellen Eye Chart
The Snellen Eye Chart was developed in 1982 by Herman Snellen, a Dutch ophthalmologist, to measure people’s visual acuity. In old days, you had to stand 20 feet away from the chart to measure your distance vision. Nowadays, digital (or mirror-based) Snellen charts are used to simulate 20 feet.
When you stand 20 feet away and be able to read only the line 1, your vision is 20/200. If you can read the line 8, “D E F P O T E C”, you have 20/20, normal vision. The line 1, 20/200, means if you have the normal 20/20 vision, then you can see the big E on line 1, from 200 feet away.
If you can read the lines below 20/20 from 20 feet away, you have what’s called “Super-normal” vision. Many airline or fighter pilots go through vision training to obtain super-normal vision (usually 20/10) to see details better in space when they fly the plane. Many people who practice healthy vision habits and correct their defective vision with natural vision improvement techniques are usually able to obtain 20/10.
I cannot even see the big E, on line 1, what do I do?
You can test your vision using the Snellen Eye Chart, from 10 feet, 5 feet, 4 feet, 2 feet, 1 feet, or 1/2 foot, etc. For example, you tested your vision from 10 feet and read the line 5 on the Snellen chart, then your visual acuity is 10/40. To convert to 20/20-based, you divide 20 by the distance you read from and then multiply that to the second number of the line you read. In this example, you divide 20 by 10, which is 2. Now you multiply 2 by 40 which gives you 20/80 as your current vision level.
What Is Diopter?
For nearsighted, the diopter has minus sign on your prescription and is equal to the reciprocal of the focal length measured in 1 meter (100 cm). For example, minus 4-diopter means your blur zone starts from (100 cm / 4) 25 cm. It’s one way of measuring the distance vision but it’s not always correct. For example, a vision student, Andrea’s blurred zone started around 30 cm. According this formula, her diopter should be around -3.33 (100/30), but her eye doctor prescribed -8.5 diopter glasses.
For farsighted, the diopter has plus sign on your prescription and refers to strength of the magnifying lens at one meter. A plus four (+4.0) diopters represent a 100% increase in magnification or 1x power magnification. A +20 diopter measurement is approximately equal to 5x power magnification.
Why do I need to convert to diopter?
Although eye doctors measure your visual acuity using simulated 20/20-based measurement for distance vision, diopter is what they use for prescription of your glasses/contacts (writen as Sphere; OD means right; OS means left). Thus, the glasses and contacts are manufactured based on diopter.
If you’re measuring your own visual acuity using the Snellen (or equivalent) Eye Chart and wondering what diopter you could use for your next pair of glasses or contacts, then you have to do some conversion.
Accuracy in How Visual Acuity Is Measured…
As you saw, it’s strange that they measure your distance vision using the Snellen Eye Chart from 20 feet (simulated) and yet the diopter is used for prescription. This is one of the reasons why the conversion from 20/20 based measurement to diopter is not straight forward.
There are also a few other factors that contribute to the difficulty of determining the accurate prescription for your corrective lenses:
- Your eyes bring the lights in and your brain interprets the image under the influence of your mind. And there’re NO tangible ways to measure your invisible mind which includes your thought, feeling, attitude, belief, and imagination.
- Your vision fluctuates from day to day, from hour to hour, and from moment to moment based on your physical, mental, and emotional state.
- If you’re getting your visual acuity measured at your eye doctor’s office, then there’re also other factors such as the lighting, the previous baseline they’re going by, who is giving you the test, white-coat fear syndrome, close proximity of tester’s face being uncomfortable (at least it was for me :), subjective questions they ask (“1 or 2?” or “3 or 4?”), etc.
No wonder why it takes a while for people to get used to their newly prescribed glasses…
Conversion of 20/20-based Measure to Diopter
The diopter range given in the table below is a very rough estimate. There’s also much variation between persons, when and where you’re wearing your corrective lenses.
Conversion Table for Distance Vision
|20/20-based Snellen||Estimated Diopter range|
|20/500||-5.00 to -6.00|
|20/400||-4.00 to -5.00|
|20/300||-3.00 to -4.00|
|20/200 line 1||-2.00 to -2.50|
|20/100 line 2||-1.75 to -2.00|
|20/70 line 3||-1.25 to -1.75|
|20/50 line 4||-1.00 to -1.25|
|20/40 line 5||-0.75 to -1.00|
|20/30 line 6||-0.50|
|20/25 line 7||-0.25|
|20/20 line 8||0.00 Normal|
|20/15 line 9||Super-normal|
|20/13 line 10||Super-normal|
|20/10 line 11||Super-normal|
Note: It’s strongly recommended that you get your eye doctors approval when you’re ready to reduce your prescription. It’s also important to wear glasses with at least 20/40 distance vision for driving. Please Stay Safe, especially in driving. When you receive glasses with weaker prescription, drive around familiar places first to be comfortable and build your confidence.
With love and seeing,
Mimi Shekoski, PhD, Natural Vision Teacher | Holistic Natural Health Doctor
Please note that teachers or coaches at Happy Eyesight do not purport to diagnose, treat or cure any eye condition or disease. Natural Vision Improvement is an educational self-help program which assists those with refractive error to train their visual system towards improved clarity and perception. Individuals must use their common sense in applying any activities or principles and take into account specific individual conditions that may be adversely affected. The information contained on this website are in addition to and are not intended to replace the care and advice of your medical professionals. Regular exams with your optometrist or ophthalmologist are important to assess both visual acuity and eye health. When using the principles or activities outlined in Natural Vision Improvement, students should always remain under the care of these health professionals. We provide the products and information on this website for your benefit, however must state that their use is entirely at your own risk.