Eye health resource
What Causes Bad Eyesight?
Learn why vision can become blurry or reduced, including refractive errors, age-related focusing changes, eye disease, childhood vision problems, and when an eye exam is important.
Reviewed for accuracy by Justin Bazan, OD on .
“Bad eyesight” is not one diagnosis. It can mean blurry distance vision, trouble seeing up close, distortion, reduced contrast, poor night vision, or vision loss that is not fully corrected by glasses or contact lenses.
The most common causes are refractive errors—problems with how the eye focuses light. But vision can also change because of age, eye disease, injury, or visual development problems in childhood.
Refractive errors are the most common cause of blurry vision
The eye’s cornea and natural lens bend light so it can focus on the retina. If the eye’s shape or focusing system does not place that image precisely on the retina, vision can be blurry.
Common refractive errors include:
- Myopia (nearsightedness): distance vision is blurry because light focuses in front of the retina.
- Hyperopia (farsightedness): the eye may need extra focusing effort, especially for near work.
- Astigmatism: differences in corneal or lens shape can make vision blurry or distorted at more than one distance.
- Presbyopia: with age, the natural lens becomes less flexible, making near focusing harder. This commonly becomes noticeable in the 40s and beyond.
Glasses and contact lenses can correct many refractive errors, although they do not change the underlying shape of the eye.
Genetics can matter—but they are not the whole story
Family history can influence the likelihood of developing refractive errors and some eye diseases. Myopia, for example, often runs in families, but environmental factors also matter. In children, time spent outdoors is associated with a lower risk of developing myopia, while genetics and near-work patterns can also contribute.
That means eyesight is rarely explained by a single “bad vision gene.”
Some eye conditions reduce vision even with the correct prescription
If vision remains reduced despite an accurate glasses or contact-lens prescription, an eye-health problem may be involved. Examples include:
- cataracts
- corneal disease or scarring
- retinal disease
- glaucoma
- optic-nerve disease
- diabetic eye disease
- macular degeneration
Some of these conditions can develop gradually and may not cause obvious symptoms early. A comprehensive eye examination evaluates more than the glasses prescription; it also assesses the health of the eyes.
Childhood vision can be affected by visual development
Children can have significant vision problems even when they do not complain of blur. Amblyopia (lazy eye) can occur when one eye—or sometimes both eyes—does not develop normal visual acuity during childhood. Strabismus, unequal prescriptions between the two eyes, and other visual-development problems can also affect vision.
Because children may assume that what they see is normal, routine pediatric eye care is important even when there is no obvious complaint.
Can screens or reading make your eyesight permanently worse?
Long periods of reading or screen use can cause temporary symptoms such as eyestrain, dryness, headaches, or intermittent blur. Those symptoms do not mean that normal screen use is “wearing out” the eyes.
In children and young adults, however, myopia development and progression are influenced by a combination of genetics and environment. Outdoor time and appropriate myopia monitoring can therefore matter even though screens themselves are not simply damaging the eyes through use.
Why can vision change with age?
Different changes occur at different stages of life. Children may develop or progress in myopia. Adults often notice presbyopia beginning in midlife. Later in life, cataracts and other age-related eye conditions become more common.
A changing prescription is often normal, but a sudden or unexplained vision change should not be assumed to be “just age.”
When should you schedule an eye exam?
Arrange an eye examination if you notice persistent blur, frequent squinting, headaches with visual tasks, difficulty driving at night, trouble reading, or a meaningful change in how clearly you see.
Seek more urgent care for symptoms such as sudden vision loss, a new curtain or shadow in the vision, flashes with a sudden increase in floaters, significant eye pain, or new double vision.
At Park Slope Eye, a comprehensive eye exam can evaluate both visual clarity and eye health. If the concern is related to an eye disease or medical condition, our Medical Eye Care page explains how those evaluations fit into care.
The key point: “bad eyesight” may be as simple as a correctable refractive error, but an eye exam is how we determine why vision is reduced and whether anything beyond a prescription change needs attention.