What is good eyesight and how is it measured ?
 
The 20/20 measurement is actually a very inaccurate measure of  your ability to see. In 1862 Dutch ophthalmologist Herman Snellen developed a standard so people had a way of measuring eyesight.

Dr. Snellen had an assistant that also was a hunter so he was considered to have good eyesight. 1 One of the experiments involved a lead type, used in those days for printing, a letter “E“ which was 8.7 m.m. high. The assistant could see the letter from 20 feet, so this became the norm 20/20. Snellen designed his own type to reflect what was considered the smallest detail a person could see.

It was considered that the smallest detail a person could see was equivalent to 5° of an arch or 1/360 of a circle. So Snellen developed an eye chart with letters where each element was the equivalent to 1° of an arch. For example, both  the black bars in the E and the white space in between was 1° of an arch. In order to determine the visual acuity Snellen graduated the chart so each line on the chart is 5% loss in your ability to see. On some charts you see the percentage notation.


What does the numbers mean?

The original Snellen equation =    Testing distance
                                                      Smallest line read

Because accurate testing of your visual acuity is 20 feet or 6 meters the reference number is either 20 for English or 6 for metric measurement. The second number indicate the distance a person with normal vision can see the letters. For example, 20/40 tells us that a person with normal vision can see these letters from 40 feet or twice as far. In metric notation the numbers are simply covered to  6/12 meaning a person can see these letters 12 meters away. This also happens to be the legal limit for driving.

Letter hight at 6 meters

Snellen notation or English units

Metric units

Decimal units

Percentage

4.4 mm

20/10/10

6/6

2

200.00%

6.5 mm

20/15

6 /4.5

1.33

150.00%

8.7 mm

20/20

6/6

1

100.00%

13.1 mm

20/30

6/7.5

0.67

90.00%

17.5 mm

20/40

6/12

0.5

80.00%

21.8 mm

20/50

6/15

0.4

70.00%

43.5 mm

20/100

6/30

0.2

60.00%

87.3 mm

20200

6/60

0.1

50.00%

174.5 mm

20400

6/120

0.05

40.00%

1

So if you can see a letter 8.7 mm high from 6 meters then you have normal vision. If you can see 4.4 mm letters at the same distance then your vision is twice as good as normal. However, there are people who can see these letters from 24 meters. People living in tribal societies often have exceptional eyesight because their lifestyle require them to look at the distance while hunting or looking after cattle. The Masai tribesmen in Kenya can see what someone is doing 1,5 kilometres away. They  probably also recognize who it is. Having lots of practice looking for stray cows enable them to develop their distance vision to its full potential. On the other hand they don't read a lot.

Actually what is normal eyesight ?

There are a few studies done to determine what normal eyesight actually is. Interestingly there are studies done in 1862 as well as more resent studies. Comparing them we find that the graph is almost identical. However, there are some surprises because we have conditioned to believe that our eyesight will fail at one point.

1

We see that normal eyesight is about 20/14 at age 20 which slowly deteriorate to 20/19 about age 75. Interestingly, old eyes only begin in your late 50's but do not get to the point where you need reading glasses before your 80's.

More than a hundred years have not causes much change in normal eyesight. What is happening is that we are over-using our visual system by looking at computers all the time of reading extensively. These activities lead to what is known as “accommodative stress” or to use a simpler term eye strain. Research shows that eye strain takes several hours to release this stress. If the stress is kept up because you are studying the tension is newer fully released and you are on your way to your first glasses.

Since there is a widespread belief that nothing can be done about failing eyesight glasses or contact lenses seems the only solution. Vision training, especially when started early on, is the natural way of restoring your vision to normal. 

If you feel you are getting out of shape you start running or going to the gym so you can restore your health. We can do the same with eye sight through Vision training.

Start early and it is easy to do.

 

................................................................Home


Menu
Bookmark and Share
What is normal eyesight