Many people understand 20/20 vision as a measure of excellent eyesight. This prompts questions about what defines less than ideal vision. Clarifying “bad” vision involves understanding specific measurements and other aspects of eye health. This article explores how vision impairment is defined and the factors contributing to it.
Understanding Visual Acuity Measurements
Visual acuity, a measure of vision sharpness, is most commonly assessed using a Snellen eye chart. During this test, a person reads letters of decreasing size from a standard distance, typically 20 feet in the United States. The resulting visual acuity score is expressed as a fraction, such as 20/20. The first number in this fraction represents the distance at which the person stands from the chart. The second number indicates the distance at which a person with normal vision could clearly read the same line of letters. A higher second number signifies less sharp vision.
What Numbers Indicate Impaired Vision
Any visual acuity less than 20/20 is considered impaired vision. For example, 20/40 vision means a person must be 20 feet away to clearly see something a person with 20/20 vision can see from 40 feet away. This level of vision is mildly impaired and can make tasks like reading road signs or small print more challenging. Corrective lenses often improve clarity, though some states may permit driving with 20/40 vision with restrictions.
Vision in the range of 20/80 to 20/100 indicates more significant impairment, affecting many daily tasks. If someone has 20/100 vision, they need to be 20 feet away to see what a person with normal vision can see from 100 feet. The threshold for legal blindness in many regions is defined as 20/200 or worse in the better-seeing eye, even with corrective lenses.
Beyond Visual Acuity: Other Aspects of Vision Health
Visual acuity is just one component of overall vision; other factors can contribute to impaired vision even if acuity is good. The field of vision refers to the entire area one can see without moving the eyes. Impairments can include “tunnel vision,” where peripheral (side) vision is significantly restricted. Another aspect is color vision, where deficiencies can lead to difficulty distinguishing certain colors.
Depth perception, the ability to judge distances and see in three dimensions, is also an important part of vision. Impaired depth perception can affect daily activities like driving or navigating uneven surfaces. Contrast sensitivity, the ability to discern objects against backgrounds of similar color or brightness, also plays a role. Difficulties with contrast sensitivity can make seeing clearly in dim light or hazy conditions challenging, even if visual acuity is otherwise good.
Common Conditions Leading to Vision Impairment
Various underlying conditions can lead to impaired vision, affecting either visual acuity or other aspects of vision. Refractive errors are common causes, occurring when the eye’s shape prevents light from focusing correctly on the retina. Myopia, or nearsightedness, causes distant objects to appear blurry, while hyperopia, or farsightedness, makes nearby objects seem unclear. Astigmatism, another refractive error, results from an irregularly shaped cornea, leading to distorted vision at various distances.
Presbyopia is an age-related condition that develops around age 40, where the eye’s natural lens loses flexibility, making it difficult to focus on close objects. Beyond refractive errors, conditions like cataracts involve clouding of the eye’s lens, which can lead to blurry or dim vision. Glaucoma, a condition that damages the optic nerve, progresses slowly and can initially affect peripheral vision. These conditions highlight that vision impairment can stem from various physical changes within the eye.