An eye chart is used by eye doctors to measure your vision. More specifically, the chart is utilized in measuring how well you see in the distance, as compared against the average population.
The Snellen Eye Chart is probably the most widely recognized and known eye chart. It was developed by a Dutch doctor named Hermann Snellen. Snellen developed his chart in the 1860’s, and although there are many variations, there are generally eleven rows of capital letters. The top row contains one letter (often a big “E”, though other letters can be used), and the other rows contain letters in progressively smaller sizes.
During an eye exam, your eye doctor will ask you to identify the smallest row of letters you can read and ask you to read it back to him or her. Those who can read the very bottom row demonstrate that they have very strong eyesight and high visual acuity.
What does 20/20 mean?
The standard placement of an eye chart is to be 20 feet away from the viewer. As most doctor’s offices are smaller than that distance, many eye doctors utilize a mirror set up to simulate a distance of 20 feet. The chart in this case actually hangs behind the patient.
As such, 20/20 describes the vision (or, more accurately, the visual acuity) of a person. It is considered “normal” vision. All it means is that you can read a letter size at 20 feet that most humans should be able to read at that distance.
Eye charts can have non-standard configurations, but generally speaking, if you can only read the largest letter (the usual “E”), you have 20/200 vision. That means you can read a letter at 20 feet that the average person can read at 200 feet. A person with 20/200 vision has poor visual acuity.
In order to have a license in most states here in the United States, one must have a visual acuity of at least 20/40. This is, of course, taking into corrective lens or contacts as needed. Those whose vision falls below 20/40 should not drive without their glasses or contacts as that is not only incredibly dangerous, but also illegal in most states.
Usually the 20/20 line is the 4th from the bottom of the chart. 20/15, 20/10, and 20/5 usually makes up the remaining three lines of the chart. Not many humans have better than 20/10 vision.
Tumbling E Chart
There are cases where the standard Snellen chart cannot be used. For example, if the patient does not speak a language that makes use of the alphabet, or if the patient is a small child who doesn’t know the alphabet yet, or if a child that is too shy to read the letters aloud, a non-Snellen chart becomes necessary.
Similarly, if a patient is illiterate, or has a handicap that renders him or her unable to cognitively comprehend and say the letters or read them out loud, alternative charts are used.
One such chart that is commonly used in this scenarios is a modified Snellen chart called the Tumbling E chart. It has the same scale as the standard Snellen chart, but all the characters are the capital E, just in different orientations (pointing in different directions, rotated in increments of 90 degrees).
The eye doctor in this case asks the patient to indicate which direction the E is pointing in – right, left, up or down.
Studies have shown there is little to no discrepancy between visual acuity measurements obtained by the Snellen chart or the tumbling E chart.
Jaeger Eye Chart
While the Snellen is useful for measuring distance visual acuity, the Jaeger Eye Chart is used for measuring your near sight. This is a small hand-held card that consists of blocks of text in various sizes. This test’s scale ranges from J1 to J11, with J1 being the smallest type. J2 is usually the “norm” or standard. Most cards are designed to be held 12 or 14 inches from your eyes.
The chart is then either held at a distance and the patient asked to read the smallest block of text possible, or the chart is moved backward and forward until you can see a certain size of text.
While eye charts are incredibly helpful in measuring your visual acuity, there are many things they do not measure. While they can determine whether you need glasses or contact lenses, eye charts do not measure your peripheral vision, depth perception, color perception, or contrast perception. It also obviously does not measure important eye health indicators such as eye fluid pressure, the dryness of your eyes or the condition of your retinas.