Tag: 20/20 vision

20/20 Vision

Karen Griffith

During an eye examination, I always use the eyechart to measure the distance vision, usually with the most current glasses or contact lenses. This gives me a place to begin and tells me what sort of vision a person has and a quantity to compare. snellen chart
The classic example of an eye chart is the Snellen eye chart, developed by Dutch eye doctor Hermann Snellen in the 1860’s. There are many variations of the Snellen eye chart, but in general it has 11 rows of capital letters. The top row contains one letter (usually an E). the other rows contain letters that are progressively smaller.

In the United States, the standard placement of the eye chart is on a wall that’s 20 feet away from your eyes. Since most of our exam rooms are shorter than 20 feet we use mirrors to simulated the 20 foot distance. 20/20 vision is considered “normal vision”, meaning you can read at 20 feet a letter that most human beings should be able to read at 20 feet.

In the US a person is considered “legally blind” if his best-corrected visual acuity with glasses or contact lenses is 20/200 or worse. To get a driver’s license in California a person must have at least 20/40 best corrected acuity.

An eye chart is a measure of visual acuity only. It is an aid to prescribing glasses and contact lenses. But eye charts don’t measure peripheral vision, depth perception, or color perception. They also don’t determine the ocular health.

The visual acuity measurements done with the Snellen eyechart are simply a starting place for any eye examination, a method to quantify it.

What is Your Vision?

Karen Griffith

SNELLENWhen patients come to see me, I need to have a way to compare how they see in relation to someone with “normal” vision. I also need to have a consistent measurement for year to year comparison. So, like most eye doctors, I use a number system called Snellen visual acuity. This measure of the clarity of vision uses black letters on a white background. This is the part of the eye examination, when I ask you to read the letters on the chart. The letters are called optotypes and they have a very specific design that takes into account the size of the lines and the space between them While there are other types of visual acuity measurements, Snellen acuity is the most common. One of the circumstances when I will use other types of tests is when I’m examining my younger patients. Then I might use shapes or broken wheel charts.Children's Acuity

The measure most people want to achieve with the Snellen visual acuity is 20/20 vision. This is considered normal. But what does 20/20 mean? Take for example a person with 20/40 vision. A person with 20/40 vision is at a disadvantage compared to a person with normal, 20/20 vision. In fact, a person with 20/40 vision would have to stand 20 feet away from something that a person with normal vision can stand 40 feet away from and still see clearly.

A number of factors like eye disease, the eye’s length and curvature, and the quality of connection between the eye and the process centers in the brain come together to determine visual acuity. Some factors, like nearsightedness, come with easy solutions like glasses or contact lenses. Others conditions like macular degeneration, are much more complex and simple solutions like glasses only offer minimal improvement.