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Distance Vision Tests

Quantifying another person’s vision is a very hard task for doctors. There are very few physical features that they can simply observe that can tell the state of one’s vision. As such, there are numerous tests that can help a doctor understand what would normally be first person information. Distance vision is perhaps the most common testable visual attribute.

The most well known visual test is Snellen chart, which is the chart with the large E on top. The test is to measure one’s distance vision. The chart is placed 20 feet away if in the US and 6 meters away elsewhere. In spaces where this type of distance is not available, mirrors are used to make the chart appear farther than it actually is. Either way, the rule of thumb should be that the distance to the chart should be 68.75 times the height of top letter. From the E, each line of letters gets increasing smaller. Each line also corresponds to an increasing visual acuity. If one cannot read the top letter with corrective eyewear, they are considered legally blind. This is 20/200. The fourth line from the bottom normally corresponds to 20/20 vision, which is considered “average” vision. Being able to read any line below that one represents an above average visual acuity.

The chart only uses 10 letters known as the Sloan letters. They are: C, D, H, K, N, O, P, T, and Z. When used in visual testing, these letters are called optotypes. A specific font must be used for the optotypes that has each letter defined with very specific measurements. A patient “passes” a line if they can accurately describe roughly 80% of the optotypes.

The Snellen chart is similar to Landolt C chart. The C optotypes in this test is identical to the one found on the Snellen chart, except the opening of the C is angled in a different way. Similarly, the size of the C’s slowly decrease and the amount of C’s increases. Again, a patient should be able to “read” 80% of the characters on each line. This time, instead of reading them aloud, the patient normally points in the direction the C is opened. This test is considered the most accurate way of assessing someone’s vision and is used in the scientific world for research involving people’s vision. A similar test is the tumbling E chart, where patients point in the direction the arms of the E are facing.

The Snellen chart has since been replaced by the logarithm of the minimum angle of resolution chart, or LogMAR. Each line logarithmically decreases in size and spacing, which means the height of the letter is cut by a factor of 10 each line. Unlike the Snellen chart, the LogMAR chart contains the same number of letters on each line. Additionally, your final score takes into account the amount of letters you missed. The patient continues reading until they cannot see the letters anymore. This line has a value that is used to calculate one’s visual acuity. 20/20 vision is a 0.0 on the LogMAR scale. Negative numbers indicate above average vision. The higher the positive number, the worse visual acuity. A 1.0 or higher is considered legal blindness. This test is employed by the UN to test visual acuity around the world.