By Liz Segre; reviewed by Gary Heiting, OD
During an eye test, eye doctors use eye charts to measure how well you see in the distance, compared with other human beings. The classic example of an eye chart is the Snellen eye chart, developed by Dutch eye doctor Hermann Snellen in the 1860s. There are many variations of the Snellen eye chart, but in general they show 11 rows of capital letters. The top row contains one letter (usually the “big E,” but other letters can be used). The other rows contain letters that are progressively smaller.
During an eye exam, your eye doctor will ask you to find the smallest line of text letters that you can make out, and ask you to read it. If you can read the bottom row of letters, your visual acuity is very good.
What 20/20 Vision in an Eye Test Means
In the United States, the standard placement of the eye chart is on a wall that’s 20 feet away from your eyes. Since many eye doctors’ offices don’t have rooms that are 20 feet long, in a smaller room the eye chart may hang behind the patient chair, using mirrors to make it appear in front of you at a simulated distance of 20 feet.
How a Snellen eye chart and a “tumbling E” chart might look at your eye doctor’s office. The tumbling E chart tests the visual acuity of young children and others who can’t read letters aloud. [Read more about children’s eye exams.]
20/20 vision (or really, 20/20 visual acuity) 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.
Eye charts can be configured in various ways, but generally, if during an eye test you can read the big E at the top but none of the letters lower than that, your vision is considered 20/200. That means you can read at 20 feet a letter that people with “normal” vision can read at 200 feet. So at 20/200, your visual acuity is very poor.
In the United States you are considered “legally blind” if your best-corrected visual acuity (meaning, your best distance vision with eyeglasses or contact lenses) is 20/200 or worse.
To get a driver’s license in most of the United States, your best-corrected visual acuity must be at least 20/40.
Usually the 20/20 line of letters is fourth from the bottom, with 20/15, 20/10 and 20/5 below that. Not many people have 20/10 or better visual acuity, but many animals do, especially birds of prey, which have been estimated to have an acuity of 20/5 or even better.
The “Tumbling E” Eye Chart
In some cases a standard Snellen eye chart cannot be used. One example is when the person having the eye test is a young child who doesn’t know the alphabet or is too shy to read letters aloud. Other examples include when the person is illiterate or has a handicap that makes it impossible for him to cognitively recognize letters or read them aloud.
In these situations, a modification of the Snellen eye chart called a “tumbling E” chart may be used. The tumbling E chart has the same scale as a standard Snellen eye chart, but all characters on the chart are a capital letter “E,” in different spatial orientations (rotated in increments of 90 degrees).
The eye doctor asks the person being tested to use either hand (with their fingers extended) to show which direction the “fingers” of the E are pointing: right, left, up or down.
Studies have shown that visual acuity measurements using a tumbling E chart are virtually the same as those obtained from testing with a standard Snellen eye chart.
The Jaeger Eye Chart
To evaluate your near vision, your eye doctor may use a small hand-held card called a Jaeger eye chart. The Jaeger chart consists of short blocks of text in various type sizes.
Many animals can see better than we can. This is an anomalocaris, a large predator that lived 500 million years ago and was recently found to have had highly acute vision.
The Jaeger type scale ranges from J1 to J11 or larger, with J1 being the smallest type. J2 is considered the equivalent of 20/20 distance visual acuity at the reading distance indicated on the card (usually 12 to 14 inches from your eyes).
The chart can be used in two different ways, depending on what your eye doctor is trying to measure:
• The chart is held at a specified reading distance (such as 12 inches) and you are asked to read the passage with the smallest type you can see.
• The chart is moved forward and back until you are able to read a certain type size.
Eye Chart Limitations
Eye charts measure visual acuity only. They do help your eye doctor figure out whether you need prescription eyeglasses or contact lenses for your distance vision. And they help the Department of Motor Vehicles to determine if you are required to use eyewear for driving, or if you shouldn’t drive at all because you are legally blind.
Watch this video on what causes blurry vision and how we can correct it.
But eye charts don’t measure your peripheral vision, depth perception, color perception or ability to perceive contrast.
And they don’t measure items related to the health of your eyes, such as your eye fluid pressure, whether you have glaucoma, how dry your eyes are or whether your retinas are in good shape.
So eye chart testing is just one component of a complete eye exam, which you should have every one or two years.
Comments are closed