The measurement of visual acuity is generally noted as the number 20 on the top and another number on the bottom of a fraction. 20/20 is considered normal vision. If the bottom number is larger than 20, the vision is worse than normal. For example, a person with 20/200 vision is seeing more poorly than someone with 20/30 vision. Some fortunate people can actually see better than 20/20 (20/15 or even 20/10). The best visual acuity thought achievable by a human is 20/8.
The minimum separation between two lines that the average person can see is 1 second of arc or 1/60 of a degree. Using a mathematical formula, this translates into an 8.75 mm (0.35 inch) letter viewed from 20 feet away. In other words, a person with 20/20 vision can read a 0.35 inch letter at 20 feet. The best a person with 20/100 vision can read at 20 feet is only a 1.75 inch letter. Stated in another way, a person with 20/100 vision would have to move into 20 feet to see the same sized letter that a person with 20/20 vision could see at 100 feet.
The visual acuity charts that we use are all based on these letter sizing calculations. Herman Snellen was the ophthalmologist that invented the first standardized eye chart in 1862. We still use similar charts to measure vision today. While our vision charts are projected or digital, we still use a mirror to simulate a 20 foot long room. So now you know what 20/20 means AND why we have mirrors in all our exam rooms.
At Madison Medical Eye Care our passion is to keep you seeing as close to 20/20 as possible for your entire life, not just in the year 2020!
Dr. James is an Optometrist practicing with Dr. Martha Jay and Dr. Lisa Bennett at Madison Medical Eye Care with offices in Mequon & Saukville, Wisconsin. He provides comprehensive eye care for the whole family and accepts most insurance plans.
For more eye care information, call 262-241-1919 or visit www.MadisonMedical.com.