Posts Tagged ‘snellen eye chart’

All about eyesight

The 20/20, is a term one would have heard so many times when it comes to discussing eyesight. What exactly is a 20/20 vision? The clarity of one’s vision is usually tested at a primary level using a Snellen eye chart. The standard method of reading the chart is from a distance of 20 feet. A 20/20 vision indicates that the person can read the chart from a distance of 20 feet and this is what is visible to a person with normal Eye Sight from 20 feet away. The same can also be termed as 6/6 vision when the distance is calculated in meters. Though 20/20 is labeled as the perfect vision one can have eyesight which is above that. The maximum clarity till recently was thought to be vision which is 20/10 but most recent studies have shown eyesight better than 20/8. There are birds such as hawks and owls which have visions that are much sharper than humans. Birds like hawks are said to have a vision which is 20/2.

There are many who do not have the so called perfect 20/20 vision which is considered most favorable by physicians. People suffer from many an eye problem like near-sightedness, far sightedness and complications such as astigmatism. 20/20 cannot be said to be the perfect eyesight but just a measure that is considered normal. It actually is only a way of denoting the vision acuity from a particular distance. It is only a basic measure of vision. But there are various other factors that affect the visual ability of a person like the side or peripheral vision, perception of depth, coordination of the eyes, the ability to focus and the color vision. All these factors together contribute to the actual visual ability of an individual.

Persons suffering from many vision problems can get the same corrected by the use of glasses or contact lenses. Some people also suffer from eye diseases which can be treated with ocular medication. A comprehensive eye examination can determine the exact problem and the necessary treatment option can be adopted. A very popular way of treating many an eye problem today is the Lasik surgery. The laser surgery is very effective and has proved beneficial to many and a large number of those who suffered from vision complications have been blessed with a 20/20 vision and Lasik surgery has proved to be beneficial and a boon to many.


Snellen Eye Chart

All of us would be familiar with the Snellen eye chart. It is the chart used to test the clarity of your vision when you visit an eye specialist. The chart gets its name from Herman Snellen, a Dutch ophthalmologist, who was instrumental in preparing the chart way back in 1862.

The chart has eleven lines in all and the first line has just one alphabet which is very large in size. In the lines below the number of alphabets increases and at the same time the size of the alphabets decreases. The test is taken in a way where one eye is covered and the person is told to read out the alphabets. The visual acuity is determined depending upon the smallest alphabet that the person is able to read. The other eye is tested in the same fashion. The alphabets shown on the Snellen Eye Chart are known as optotypes. The letters are not printed in an ordinary manner but the length, width and height do have some geometrical implications.

The customary Snellen chart has only the following 10 alphabets C, D, E, F, L, N, O, P, T, Z. The chart that is wall mounted is inexpensive and is generally used for a preliminary examination of the eye. The Visual acuity is determined by a formula which is:

Distance from where the test is conducted/ distance where the smallest optotype recognized subtends at an angle of 5 arcminutes. The chart is placed at a standard distance of 20 feet in most countries for the test. The chart also has certain specifications, it should be made on non-reflective material, it should have a matte finish, should be made of durable plastic and also the standard size is 22”*11”.

The Snellen eye chart also has its variations i.e. for illiterate persons and for young children who do not know how to read. For the illiterate, the chart has the alphabet E written in various directions and the ones for young children have symbols on it. The Snellen eye chart has also been a subject for a lot of criticism. Many are of the opinion that patients might memorize the chart and this would end up in a wrong result. Also after testing one eye they may be able to memorize it because of which the other eye may seem perfect. But even with all the criticism the chart has been in use all these years. Even with so much progress made in the field of medicine, the Snellen Eye Chart is the primary test one has to go through which speaks volumes about it.

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