So what does 20/20 vision mean?
<p>If you’ve ever wondered what 20/20 vision actually means, we’ve got the lowdown here. When you have your eyes tested at the optometrist, they are measuring how well you can see in the distance compared to other people. Traditionally, optometrists use the Snellen eye chart, which was developed by Dutch eye doctor Hermann Snellen in the 1860s.</p>
<p><strong>The Snellen eye chart</strong></p>
<p>Though there are a few variations, the chart generally shows 11 rows of capital letters. As you go down from the top row, the letters become smaller and smaller.</p>
<p>During your test the optometrist will ask you to keep reading down the chart until you can no longer make out the text on a particular line of letters. If you can read the bottom row (which many people can’t), then your visual acuity is considered very good.</p>
<p>You may have heard of the expression “20/20 vision” and wondered what that means. The eye chart is normally placed on a wall 20 feet away from your eyes. Most optometrists don’t have rooms quite that big, so they use a combination of mirror and chart placement to simulate the 20-feet distance.</p>
<p>If you have 20/20 vision you can read a letter at 20 feet that most people should be able to read at 20 feet. This is considered normal vision.</p>
<p>At the other end of the spectrum, if you can only read the very top letter on the chart (it’s normally an E) your vision is considered 20/200. Meaning you can read a letter at 20 feet that people with normal vision can read at 200 feet.</p>
<p>As a general rule, the 20/20 line of letters on the chart is on the fourth row from the bottom, with 20/15, 20/10 and 20/5 below that. While it’s not common for people to have visual acuity of 20/10, many animals do. Birds of prey have been estimated to have an acuity of 20/5 or even better.</p>
<p><strong>The Tumbling E eye chart</strong></p>
<p>There are some instances where the Snellen eye chart is not appropriate. For instance if the patient is a child who can’t yet read or might be a bit shy, another chart is used.</p>
<p>A modified Snellen chart, called the “tumbling E” chart may be used instead. While it has the same scale as the Snellen eye chart, all of the characters used on the chart are variations on the capital letter “E” – where it is rotated by 90 degrees to form different shapes.</p>
<p>Instead of asking them to “read” the text, the optometrist will ask the patient to show the direction of the E on their hands. The measurements have been shown to be almost the same as when the Snellen eye chart is used.</p>
<p><strong>The Jaeger eye chart</strong></p>
<p>When it comes to your near vision, an optometrist might use a small hand-held card called a Jaeger eye chart. This chart has short blocks of text in various type sizes.</p>
<p>In the same way as the far sight tests, the Jaeger type scale allows the optometrist to measure the equivalent of 20/20 vision on the card – this is generally around 12 to 14 inches from your eyes. </p>
<p><strong>Eye chart limitations</strong></p>
<p>An eye chart can help your optometrist determine whether you need glasses or contact lenses; and it can also help the RMS decide whether you need to use eyewear while driving (or whether you can’t drive at all due to vision impairment).</p>
<p>But an eye chart can’t measure things such as your peripheral vision, your depth perception, or your ability to perceive colour or contrast. They also don’t measure the health of your eye, in terms of glaucoma, fluid on the eye, dryness or the health of your retina.</p>
<p>These are all looked at when you have your eye exam. Ideally you should try and have your eyes checked every one or two years.</p>
