Presbyopia
As people age, the lens lose the ability to
accommodate as well as they could when they were younger. This condition,
presbyopia, begins affecting people in their forties or fifties, causing
people to lose their clear near vision. This condition is another
refractive disorder.
Causes:
Age is usually the cause of this disorder. The lens
becomes less flexible with age, and therefore encounters more and more
difficulty trying to focus on objects up close. The hardening lenses of
the eyes causes vision up close to be blurrier and blurrier. The average
10 year old can still focus on objects just 3 inches away from the eyes.
This closes point at which an object can be viewed in focus is called the
near point of accommodation. At age 12, this point has decreased to
about 4 inches. By age 40, the distance has increased up to 6 to 10
inches. This increases to around 39 inches by the time a person reaches
60 years old. This is why most old people can't read without glasses.
Correction:
Glasses or contacts are commonly used to correct
presbyopia. People who needed glasses when younger now may need 2 pairs,
one used for seeing things far away, and the other to see things up
close. Oftentimes people use bifocals instead of two pairs of glasses,
with the top portion used for distance viewing and the lower for reading.
Bifocal contacts have a central circle for distance viewing, while the
surrounding area contains the prescription for reading.
