Defective Vision

Defective vision is a broad term that usually refers to farsightedness (hyperopia) and nearsightedness (myopia) but color blindness, astigmatism, and night blindness are also considered to be categories of defective vision.

Causes and Risk Factors

The causes and risk factors of defective vision depend on the type of vision defect that the individual is suffering from.

Farsightedness or hyperopia (hypermetropia)

Individuals who are suffering from farsightedness can see objects at a distance fairly clearly but struggle to see objects that are close. For instance, a farsighted individual may be able to see clearly enough to drive while needing corrective lenses to read a book.

Farsightedness is a genetic trait that results in a cornea that is flatter than normal or with a very short distance between the cornea and the retina. These factors cause images to be focused at a point that is beyond the retina, which means that the distance vision remains clear but the objects that are close appear to be blurry. Farsightedness becomes a degenerative disorder as age increases so most individuals who are older than 40 years old lose the ability to focus their lenses. In some rare cases, diseases such as retinopathy (degeneration of the retina), eye tumors, and lens dislocation may also lead to farsightedness.

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Nearsightedness or myopia

Individuals who are suffering from nearsightedness find it difficult to clearly see distant objects. The near vision is reasonably clear so individuals can still identify objects that are located closely.

Like farsightedness, nearsightedness is typically caused by hereditary conditions. When the eyeball is longer than normal, the image is focused in front of the retina rather than on it so nearsighted individuals can see close objects fairly clearly while distant objects appear blurrily. This condition is usually detected during childhood and progresses over the following years until it stabilizes during adulthood.

Color blindness

Colorblindness is primarily a congenital defect and it is a condition in which individuals have mild to severe difficulty with identifying colors. Color-blind people find it difficult to recognize different shades of colors and, in some extreme cases, cannot recognize some colors at all. Colorblindness is caused when the cone cells, whose function is to identify colors, are absent in the retina of the suffering individual. More males are affected by color blindness than are females. Some of the additional causes or risk factors of colorblindness are as follows:

Chronic illnesses such as Alzheimer's disease, diabetes, glaucoma, leukemia, liver disease, alcoholism, multiple sclerosis, and retinitis pigmentosa.
Accidents or strokes.
Side effects of medicines that are used to treat tuberculosis, high blood pressure, and nervous disorders.
Exposure to hazardous chemicals such as carbon monoxide, carbon disulfide, fertilizers, and lead-based chemicals.
Aging (risk of colorblindness increases after the age of 60).


Astigmatism is a type of refractive disorder that causes blurry vision. It occurs when the front surface of the cornea or the lens has uneven surface curvature. A distorted cornea causes corneal astigmatism and a distorted lens causes lenticular astigmatism. This condition may be congenital and may become aggravated when the individual also suffers from nearsightedness or farsightedness. Astigmatism may also develop after eye injury, surgery, or disease.

Night blindness

Night blindness affects the vision at night or when there is little or no light. Some of the common causes of night blindness are cataracts, retinitis pigmentosa, vitamin A deficiency, certain medications, birth defects, and uncorrected myopia

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