If you have an eye exam and are told you have 20/20 vision, does this mean you have perfect eyesight? Is it possible to achieve even better than 20/20 vision? To answer these questions, let’s take a closer look at vision-related terminology to fully understand how eye doctors measure the quality of your vision. Thanks to Amy Hellem at All About Vision we have those answers.
Visual acuity, eyesight and vision: What’s the difference?
Visual acuity. This, literally, is the sharpness of your vision. Visual acuity is measured by your ability to identify letters or numbers on a standardized eye chart from a specific viewing distance.
Visual acuity is a static measurement, meaning you are sitting still during the testing and the letters or numbers you are viewing also are stationary.
Visual acuity also is tested under high contrast conditions — typically, the letters or numbers on the eye chart are black, and the background of the chart is white.
Although visual acuity testing is very useful to determine the relative clarity of your eyesight in standardized conditions, it isn’t predictive of the quality of your vision in all situations. For example, it can’t predict how well you would see:
- Objects that are similar in brightness to their background
- Colored objects
- Moving objects
Three major physical and neurological factors determine visual acuity:
- How accurately the cornea andlens of the eye focus light onto the retina
- The 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
Only light that is focused on a very small and highly sensitive portion of the central retina (called the
Visual acuity typically is quantified with Snellen fractions (see “What is 20/20 Vision?” below).
Eyesight. The exact definition of “eyesight” is difficult to pin down. Depending on which dictionary or other resource you check, it can mean “ability to see,” “the sense of seeing,” “vision,” “range of sight” or “view.” Often, the terms “eyesight” and “visual acuity” are used interchangeably.
Vision. This is a broader term than visual acuity or eyesight. In addition to sharpness of sight or simply a description of the ability to see, the term “vision” usually includes a wider range of visual abilities and skills. 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.
If this more inclusive (and accurate) definition of “vision” is used, what most people call “20/20 vision” should really be called “20/20 visual acuity.” Realistically, that probably won’t happen. For better or worse, the term “20/20 vision” is likely here to stay.
What is 20/20 vision?
The term “20/20” and similar fractions (such as 20/40, 20/60, etc.) are visual acuity measurements. They also are called Snellen fractions, named after Herman Snellen, the Dutch ophthalmologist who developed this measurement system in 1862.
In the Snellen visual acuity system, the top number of the Snellen fraction is the viewing distance between the patient and the eye chart. In the United States, this distance typically is 20 feet; in other countries, it is 6 meters.
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 â€” 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 increasingly larger letter sizes on the lines on the Snellen chart above the 20/20 line correspond to worse visual acuity measurements (20/40, 20/60, etc.); the lines with smaller letters below the 20/20 line on the chart correspond to visual acuity measurements that are even better than 20/20 vision (e.g., 20/15, 20/12, 20/10).
The single big “E” at the top of most Snellen eye charts corresponds to 20/200 visual acuity. If this is the smallest letter size you can discern with your best corrective lenses in front of your eyes, you are legally blind.
On most Snellen charts, the smallest letters correspond to 20/10 visual acuity. If you have 20/10 visual acuity, your eyesight is twice as sharp as that of a person with normal (20/20) vision.
At Schmidt’s Optical and Hearing™ in Stuart Florida let one of our licensed opticians or well trained technicians help you find your next perfect pair of glasses to get you back to “20/20” and discover why we’ve kept our patients happy for over 40 years.