The charts used for the diopter assessment usually contain several rows of random letters. The top row starts with very large letters and gets smaller and smaller as you get closer to the bottom of the chart.
What does identifying these letters have to do with glasses? Each row of the chart has been assigned a number. During the test, patients read each row of the chart until the letters become too small to read easily.
If you can read line 20 from a distance of about 20 feet, that means you have 10/10 vision. Doctors consider this normal vision, for which there is no need for glasses. In some cases, people can only see the giant letter at the top of the graph. This means they have 1/100 vision. People with 1/100 vision are considered legally blind. Figuring out which end of these two ends you are at is the purpose of the optical evaluation.
Why wear glasses?
There are many different medical reasons why people wear glasses. The simple answer is that glasses help people see better. Some people wear glasses to read things that are far away or to drive. Other people wear glasses only when they are doing a close-up activity, such as reading, writing, or knitting.
Some people wear glasses all the time. People who have trouble seeing distant objects have myopia. This means that they can easily see objects that are close to them. When seeing objects in the distance, however, they have more difficulty.