An eye chart, also called a Snellen chart, shows letters at a progressively smaller size. It is used by optometrists to measure visual acuity in patients. It was developed by Dutch ophthalmologist Herman Snellen in 1862. However, nowadays many eye doctors use a LogMAR because it is more accurate. 20/20 vision describes normal vision at a 20 feet distance from the chart. Patients who have 20/20 vision are able to clearly make out letters and objects from a distance of 20 feet, that a person with normal vision is able to see from the same distance. If a person has 20/100 vision, it means that they can only make out objects from 20 feet away that someone with “normal” vision would be able to make out clearly from a distance of 100 feet. This means they have very poor vision.
An eye chart, also called a Snellen chart, shows letters at a progressively smaller size. It was developed by Dutch ophthalmologist Herman Snellen in 1862. However, nowadays many eye doctors use a LogMAR because it is more accurate.
Is 20/20 Vision Perfect?
It is a common misconception that 20/20 vision is “perfect.” In fact, it is only “normal.” If a person has 20/15 vision, for example, they can clearly make out objects from 20 feet that a person with 20/20 vision would need to be 15 feet away from to see clearly. 20/20 is the vision required for precise tasks like reading stock quotes in the newspaper or becoming a fighter pilot, however better vision is possible and measurable.
The standards for visual acuity vary by jurisdiction. In some places, 20/40 vision is considered the minimum to drive a car without corrective lenses, for example. In the province of Ontario, drivers must have visual acuity of 20/50 or better. If you have worse visual acuity than this, you will need to visit your optometrist for corrective lenses. Patients with vision of 20/200 (one-tenth of normal vision) are considered to be legally blind.
What Eye Charts Don’t Measure
Remember, eye charts are only a rough measure of visual acuity. They do not measure depth perception, peripheral vision, colour perception, or ability to perceive contrast. For example, someone could have 20/20 vision when staring directly ahead. But if they have less than 20 degrees of peripheral vision, they are considered legally blind.
Furthermore, in no way do eye charts measure the health of your eyes. Other tests are needed to measure your eye fluid pressure, eye dryness, retinal health, and presence of eye diseases like glaucoma.
This is why eye charts are only one component of an eye exam to assess your overall visual and optical health. Contact Laurier Optical Innes Eye Clinic today to schedule your next eye exam.