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What Is Visual Acuity? - Definition, Scale & Tests.
What Is Visual Acuity? - Definition, Scale & Tests
What Is Visual Acuity?
Have you ever stood in a doctor's office and had to read from a small chart across the room? Usually there's a large letter 'E' at the top, and as you move down the chart, each row of letters gets smaller and smaller. The doctor might have you repeat this exercise many times, covering one eye or reading it with both eyes open.
This particular chart is called a Snellen chart, and it is one test used to measure visual acuity. Visual acuity refers to how sharp or clear your vision is. Each time you see an eye doctor, called an ophthalmologist or optometrist, they are measuring your visual acuity.
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Measuring Visual Acuity
Once you read a Snellen chart, how does the doctor know what to do with the results? Well, the results are usually compared to a standard called 20/20 vision. If you have 20/20 vision, it means that, at a distance of 20 feet, you can see things as clear as the average person. Obviously, this is a slightly subjective process, but it allows the vision of different people to be approximately compared to a scale.
As you might be familiar with, sometimes a person's vision changes from this 20/20 standard. If you can see things at a distance very clearly, but have trouble focusing on things close to you like the text of a book, you have hyperopia , also called farsightedness. Conversely, if you can see things up close clearly but can't focus on objects that are at a distance, you have myopia, also called nearsightedness. Sometimes people may have trouble focusing on things that are both near and far.
By analyzing your visual acuity, a doctor can estimate how sharp your vision is and then adjust it with the use of corrective lenses to help bring you back to the 20/20 standard.
Snellen Chart Examples
Let's take a look at the Snellen chart above. On the left of the chart, you can see each row of letters that get smaller and smaller as you move down the chart. The lines are numbered, and then there's a number similar to 20/20 on the right. How does this work?
The doctor's office will have a spot marked on the floor where you are to stand to complete the exercise. You will probably cover one eye and then begin reading down the chart as far as you can, until the letters become too small to read. With this chart, if you can read through line 8 without any problems, you have 20/20 vision. Congratulations! That's the standard we talked about above.
However, if you can only read through line 4, you have 20/50 vision. What does that mean? A measure of 20/50 vision means you have to stand at a distance of 20 feet to read what most people can read from 50 feet. This is one version of nearsightedness; you have a harder time seeing things farther away.
What if you can only read the top line? Well, this means your visual acuity measure is 20/200. You have to stand at a distance of 20 feet to see what most people can see from 200 feet away! That means you are even more nearsighted than the person with a visual acuity measure of 20/50.
But wait, the Snellen chart we have here has 11 lines - what if a person can clearly read all the way through line 11? This means their visual acuity is better than 20/20. Though there aren't numbers listed here, it might be something like 20/10, meaning they can stand at 20 feet and see something clearly that most people have to be 10 feet closer to see. That's some good visual acuity!
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