The last time you were at the optometrist’s office or visiting your ophthalmologist, were you subjected to a series of eye exams? Well, during an eye test, your doctor probably used a few different eye charts to measure how well you see into the distance – and up close – when compared with other people. The best example: the Snellen eye chart – which was developed by Hermann Snellen in the 1860s. While there are quite a few variations of this eye chart, they typically consist of 11 rows that contain all capital letters. You’ve probably seen them – the top row usually has a big ‘E’ and then the following rows get progressively smaller. Think back to your last exam, were you asked to read the bottom row of letters? If you did so successfully, congratulate yourself! Your visual acuity is pretty good!
You may have come across the term 20/20 vision at some point in your life, right? Well, in the U.S., the standard eye chart is placed on a wall that is 20 feet away from your eyes. A lot of doctors don’t have rooms that large, so in a smaller room the Snellen eye chart may be placed behind you. Mirrors are then used to make it appear as if it’s 20 feet from you. If you’ve been told you have 20/20 vision, it’s considered normal vision and you can read every letter on the chart from a distance of 20 feet. In theory, most people should be able to read at 20 feet. Yet, if you can’t read any letter except that big ‘E’, your vision is considered low at 20/200 – meaning you can read at 20 feet a letter that people with normal visual acuity can read at 200 feet. In the U.S., you’re considered legally blind if you can read at 20/200 vision or worse.
There are other charts used during eye exams, like the Tumbling E eye chart, which is used when the standard Snellen chart can’t be used – usually with a child who doesn’t know the alphabet yet or is too shy to read the letters out loud. This chart uses the same scale as the Snellen eye chart but the characters are all a capital E but they are rotated in different directions in increments of 90 degrees.
Another chart used to evaluate your vision is the Jaeger eye chart – which is actually used to for near vision. The Jaeger chart is a series of short blocks of text that come in various type sizes. This chart has a scale that ranges from J1 to J11 or more. J1 is the smallest type and J2 is considered to be the same as 20/20 vision at the reading distance that is indicated on the card. The Jaeger chart is held at a specific reading distance – usually around 12 inches – and you are asked to read the passage on the card at the smallest type you can see. The card is then moved backwards and forwards until you can read a certain type size.
Eye charts are somewhat limited and they are only meant to measure your visual acuity – making them just one part of your complete eye exam for prescription glasses or contact lenses. Eye charts do not check your depth perception, how you view color and contrast, or your peripheral vision. They also won’t measure any other items related to your eye health – like eye fluid pressure. They also can’t check to see the symptoms of an oncoming visual disease. However, they are an integral part of your eye exam as they check to see if your eyeglasses prescription changed at all. They’re a great way to test how well you see and from what distance.