The visual index
We are all familiar with the common concept of "seeing 6/6" and are sure it represents excellent vision, but is this perception really true?
What does it look like to be excellent?
How is normal vision defined and what tests are included in it?
And what does the number 6 in 6/6 represent?
It's time to make some order.
There are different characteristics for defining the quality of our vision: field of view, color vision, contrast, depth perception and more.
One of the important characteristics is visual acuity.
The more separate, the sharper
One of the definitions for visual acuity is Minimum Separable - the optical ability to distinguish between two items.
The Dutch ophthalmologist Hermann Sanlan (1908-1834) stated that a person's normal separation ability between two items is when there is an interval of one minute between items - a key angle of view of one arc minute (1/60 degree).
That is, someone whose visual acuity is less good than a single-arc minute viewing angle will not be able to distinguish between two different items and will see them as one.
This definition is similar to the resolution terminology familiar to us from the world of digital images.
The more we can distinguish between the different items that make up the image, we actually see more pixels, the image will look clearer and sharper and it is said that its resolution is higher.
E can read it
The ideal letter for identification on the Sanlan board is the letter E. This letter can be arranged evenly and symmetrically, into five parts equal in size of one arc minute per part.
It can be seen that the letter E is divided into five squares.
Each square is equal to the angular size of a minute a person with a resolution of 2 minutes of arc, ie 2 times less than the normal set by Sanlan, will not see the separation between the lines so that the letter E will look like one black rectangle
How do you put 6 meters into short rooms?
The usual test distance for examining the visual acuity of a distance is six meters, when optically the meaning of a test from this distance is similar enough to examining the vision from infinity.
Due to the size limit of the test rooms, which are probably not long enough, there are alternative methods that work according to the same principle, such as the "mirror method" - a mirror located three meters from the subject usually.
When the subject looks in the mirror he sees the letters on which he is looking from a distance equal to six meters.
Snlan's fracture
Our visual acuity is measured in the form of a fracture called the "senile fracture" and is defined as follows:
It can be described that the role of the fraction is to compare the seeing ability of the tested person represented in the numerator to the seeing ability of the normal average person who sees from a distance of six meters and is represented in the denominator.
for example:
• A person with 6/6 vision will clearly identify a specific object from a distance of six meters - as the normal person sees
• A person with 6/12 vision will clearly recognize the same specific object only from a distance of three meters - 2 times less good than the normal person
• A person with 6/3 vision will recognize the specific object well from a distance of 12 meters - 2 times better than the normal person
The distance and size do determine
In other words, visual acuity depends on two parameters: distance and size.
The larger the distance at which objects are seen and the smaller they are, the smaller the spatial angle of view (measured in arc minutes) and the better the visual acuity.
Vision 6/6, according to a simple trigonometric calculation, means - vision of an item measuring 8.72 mm from a distance of six meters.
Sounds amazing, sounds great, sounds like this is the ultimate visual acuity?
So it's not!
Turns out 6/6 is not the best there is
As we understand, the 6/6 setting reflects average and normal visual acuity, but in practice some people are able to see even better.
A study published in the USA in 1996 showed that the best visual acuity found in a human, according to Sanlan's board, was 6 / 2.66 - 2.25 times better than that of a normal person.
In addition, we saw that the quality of vision depends not only on visual acuity, but also on other characteristics such as contrast, field of view and more.
The Sanlan board examines only one characteristic out of all the characteristics, which can define our vision as "excellent".
To illustrate, imagine what would happen if we saw the "step caution" sign in the distance - but did not see the step itself, due to a defect in our field of vision.
Nor would we be proud of our sharp vision, if we look at each object in a double way.
Most of us sometimes tend to answer the question "How are you?"
In a concise answer: "Excellent", so also in the field of optics we tend to round corners and settle for a general answer.
Next time you are asked "How do you see?"
Feel free to convert the answer to Sanlan's fraction and answer: "I see 6 / 2.66".
The author of the article is Elad Lefkipker, Optometrist, Super-Pharm Optic Mevaseret Mall
