Vision scores are a standardized way to measure how well a person can see, primarily focusing on distance visual performance. While vision is a complex sense involving multiple abilities, the most common measurement people refer to is visual acuity, which measures the sharpness of sight. A “good” vision score is therefore not just a single number, but a combination of several factors that determine the clarity and function of the eye. These measurements set the stage for understanding a person’s ability to perform daily tasks and for diagnosing potential eye conditions.
Understanding Visual Acuity Scores
The primary method for quantifying distance vision is the Snellen fraction, developed in the mid-19th century. This fraction, commonly expressed as 20/X, represents a comparison between a person’s visual performance and what is considered the standard for normal sight. The numerator, which is almost always 20 in the United States, indicates the distance in feet at which the patient is standing from the eye chart during the test.
The denominator represents the distance in feet at which a person with statistically normal vision can read the same line of letters. For example, a score of 20/40 means the patient must stand 20 feet away to clearly see a letter that a person with normal vision could identify from 40 feet away. The score of 20/20 is the benchmark for normal visual acuity.
Scores with a larger denominator, such as 20/100, indicate poorer visual acuity, requiring corrective lenses to improve the sharpness of vision. Conversely, a score of 20/15 is considered better-than-normal visual acuity. This means that at 20 feet, the patient can read a line of text that a person with 20/20 vision would have to move to within 15 feet to see clearly.
Classifying Vision Levels
Visual acuity scores are used to classify vision into clinical categories, which often determine the need for correction or specialized services. Clinicians distinguish between a person’s uncorrected vision and their “best corrected” acuity, which refers to the sharpest vision a person can achieve while wearing glasses or contact lenses.
Vision between 20/20 and 20/40 is generally considered within the normal range for most daily activities, including driving in many jurisdictions. When vision cannot be corrected to better than 20/70 in the better eye, it is often classified as low vision. Low vision describes a significant visual impairment that cannot be fully corrected with conventional means, making daily tasks difficult.
A more severe classification is legal blindness, which is a specific threshold used by government agencies to determine eligibility for benefits and services. In the United States, this status is defined by a best corrected visual acuity of 20/200 or less in the better eye. Legal blindness can also be met if a person has a severely restricted visual field, sometimes called “tunnel vision,” where the widest diameter is 20 degrees or less.
Being legally blind does not mean a person has total blindness, as most individuals in this category still have some usable sight, such as being able to perceive light or shapes. The acuity score provides the framework for this classification, moving from mild visual impairment to profound impairment based on the specific fraction achieved.
Other Essential Measures of Vision Health
A high visual acuity score alone does not guarantee overall good vision, as visual health encompasses several other functions.
Color Vision
Color vision testing identifies deficiencies in distinguishing colors, such as red-green color blindness. This test is important because certain occupations, like piloting or law enforcement, have strict requirements for accurate color perception.
Peripheral Vision
Peripheral or visual field testing assesses the entire area a person can see without moving their eyes. A normal visual field extends approximately 100 degrees temporally, 60 degrees nasally, 60 degrees superiorly, and 70 degrees inferiorly. This test is important for detecting conditions like glaucoma or neurological issues, which can cause subtle loss of side vision.
Depth Perception
Depth perception determines a person’s ability to accurately judge the distance between objects. This ability relies on binocular vision, which is the coordination of both eyes working together to create a single, three-dimensional image. Poor depth perception can affect tasks such as driving, catching a ball, or navigating uneven terrain.