Snellen Eye Chart FAQs

The so-called “Snellen chart” is an eye chart that is used to measure visual acuity, or how well somebody sees. Snellen charts are named after a Dutch ophthalmologist, Hermann Snellen, who developed the chart more than 150 years ago in 1862.

People refer to tests with an eye chart by different names including vision screening or vision testing with an eye chart, an E chart (because the largest letter at the top of the chart is typically an E), a Snellen test, a visual acuity test, a Snellen visual acuity test or chart, a Snellen eye chartor simply an eye test.

Yes. While the Snellen chart is the most prevalent, there are other charts, including the tumbling E chart; a Lea symbols chart or test, named for Finnish pediatric ophthalmologist Lea Hyvärinen; a Landolt C chart (also known as the Japanese Vision Test, the Landolt ring or the Landolt broken ring) which was developed by the Swiss-born ophthalmologist Edmund Landolt.

A Snellen chart is placed at a standard distance of 20 feet away in the US (or 6 meters in the rest of the world). A person is then asked to read the letters on each line, starting at the top. If they can read the smallest line that a person with normal acuity can read at a distance of 20 feet, then they are considered to have 20/20 vision. If, however, they do not have normal vision and some of the smaller lines are blurry, then their visual acuity is determined by the smallest line they can read. For example, a person who has 20/40 vision, determined by using a Snellen chart or Snellent test, means that they can only read at 20 feet the same letters that a person with “normal” vision would be able to read at twice the distance, or 40 feet away. The largest letter on a Snellen eye chart represents a visual acuity of 20/200, which is sometimes referred to as being “legally blind.”

It depends. Eye charts have several limitations. For one, using an eye chart to measure visual acuity requires a verbal response from the child, so it is not appropriate for preverbal children. In addition, properly administering an eye test requires a child to have each eye tested individually and not “cheat” by peeking through the closed eye or try to remember the order of the letters from the first eye tested. In addition, while studies have shown that a test with an eye chart may reliably detect vision problems caused by myopia in adolescents, but not hyperopia or astigmatism, according to a 2010 study in the Archives of Ophthalmology. (Leone J,Mitchell P,Morgan I,et al. Use of visual acuity to screen for significant refractive errors in adolescents: Is it reliable? Arch Ophthalmol.2010;128(7):894-899). Eye charts also may not be able to reliably detect strabismus or cataracts or other media opacities, which can be detected by photoscreening or other instrument based screening.

Snellen Chart FAQs
These FAQs discuss the Snellen Eye Chart (also called the Snellen chart, Snellen test, Snellen visual acuity test, Snellen vision chart, or just Snellen).