Many people consider 20/20 vision to be perfect. You may be asking, “Is this accurate? Can someone have better vision than 20/20?” In order to understand these questions, one must first understand the various eye tests available, as well as the definitions of visual acuity, eyesight and vision.
There are several different types of vision tests that may be administered:
- Refraction test: this test shows your level of refractive error and finds out the right prescription for glasses or contact lenses. Both nearsightedness and farsightedness occur when light rays entering the eye can’t focus exactly on the retina. These tests can also be used to screen for eye disease.
- Color vision tests: these tests check your ability to determine colors. They are used to screen for color blindness.
- Visual field tests: these tests check for gaps in peripheral vision. They are used to screen for eye diseases, such as macular degeneration and glaucoma, which cause gaps in the visual field. They may also be used to look for damage to the nerves of the eye following a stroke, head injury, or other condition that causes reduced blood flow to the brain
- Visual acuity tests: these measure the sharpness of your vision, and are outlined below.
The sharpness of your vision is referred to as “visual acuity”. It is measure by your ability to identify letters on a standard eye chart from a specified distance. These tests are static, meaning both the patient and the eye chart stay in one place during the exam. The eye chart replicates high contrast situations, with black letters on a white background. Visual acuity testing can be useful to determine the relative clarity of your eyesight. However, it cannot predict how well you would see objects that are similar in color to their background, moving objects or colored objects.
Three major factors determine visual acuity:
- The health of the cornea when focusing light onto the retina
- The health and sensitivity of the nerves in the retina and vision centers in the brain
- The ability of the brain to interpret information received from the eyes
“Eyesight” generally means “the ability to see.” “Eyesight” and “visual acuity” are used interchangeably by most people.
“Vision” usually refers to a broader set of abilities beyond “the ability to see”. These include contrast sensitivity, the ability to track moving objects with smooth and accurate eye movements, color vision, depth perception, focusing speed and accuracy, and more.
What is 20/20 Vision?
When people use the term “20/20 vision” they are actually referring to “20/20 visual acuity”, as it reflects acuity measurements. The fractions used to denote visual acuity measurements are Snellen fractions, named after Herman Snellen, the Dutch ophthalmologist who developed this measurement system in 1862. In these fractions, the top number is the viewing distance between the patient and the eye chart. This distance is typically 20 feet in the United States.
At this testing distance, the size of the letters on one of the smaller lines near the bottom of the eye chart has been standardized to correspond to “normal” visual acuity — this is the “20/20” line. If you can identify the letters on this line but none smaller, you have normal (20/20) visual acuity.
The larger letter sizes correspond to “worse” visual acuity measurements such as 20/40, 20/60, etc. The large letter “E” on the top line corresponds to a visual acuity of 20/200., A person is considered legally blind if this is the smallest letter a person can see. Vision better than 20/20 is possible. Many young people with healthy eyes can identify letters on the 20/15 line or smaller.
Sometimes, people with 20/20 vision feel as if their vision is not as sharp as they would like. If this is the case, discuss your options with your eye care provider.
If you are in need of an eye exam to determine your visual acuity, book your appointment today!