What is 20/20 vision?
We measure visual acuity with a standardized chart called a Snellen Eye Chart. It is familiar to most people who have had a vision test. It consists of letters decreasing in size, usually with a big E at the top. The test is normally done at a distance of 20 ft. One eye is covered and the person is asked what is the smallest line on the chart they can read, and then the other eye is checked. 20/20 is a term used to define ‘normal’ vision, based on seeing a certain size image on the chart. If you have 20/20 vision, you can see clearly at 20 feet the size image that most people with ‘normal’ vision see at that distance.
If you have 20/100 vision, it means you must go as close as 20 feet to see what a person with normal vision can see from 100 feet back. Visual acuity measurements are used for certain professions. For example, a pilot needs 20/20 corrected vision to fly. To get a driver’s licence the requirement is usually 20/40 in the best corrected eye. A person is considered legally blind if the best vision for them, even with spectacles, is 20/200.
What about children who don’t
know letters or can’t respond?
There are two charts that can be used in these instances. One is a chart with pictures, and the other is called the tumbling E chart.
The first one has easily recognizable pictures that people will know. The second has the letter E in various positions and the person has to tell the examiner in which direction the E is pointing (left, up, etc) .
Do most people have 20/20 vision?
Most people do not have 20/20 vision, but can be corrected to 20/20 by the use of spectacles. In addition, you may have different visual acuities in each eye. Some of the reasons for decreased visual acuity are:
1) Myopia, or nearsightedness, where near objects are seen clearly, but far objects are not. Nearsightedness occurs if your eyeball is too long or the cornea has too much curvature, so the light entering your eye is not focused correctly. Visual acuities for nearsighted people without glasses can range from 20/25 to 20/400. A sign of nearsightedness is difficulty seeing distant objects like a TV screen or a blackboard.
2) Hyperopia, or farsightedness, where distant objects are seen clearly, but close ones are not. Farsightedness occurs if your eyeball is too short or the cornea has too little curvature, so light entering your eye is not focused correctly. Common signs of farsightedness include difficulty in concentrating and keeping a clear focus on near objects, eye strain, fatigue, headaches after close work/reading, or aching or burning eyes.
3) Astigmatism which occurs when the front surface of your eye, the cornea, is slightly irregular in shape. Think of it being egg shaped instead of round like a ball. This irregular shape prevents light from focusing properly on the back of your eye, the retina. As a result, vision may be blurred at all distances. Most people have some amount of astigmatism.
An optometric examination will include testing for all forms of visual impairment. If needed, your optometrist can prescribe eyeglasses or contact lenses to change the way the light enters your eyes and to focus the images on the retina. Glasses do not correct your eyes, nor do they make your eyes worse or dependent on them; they just help you to see better.
