3. Defects of Eye - II

Presbyopia:

The ciliary muscles weaken with age, and the eye-lens become rigid (inflexible), causing the eye to lose its ability to accommodate. As a result, an older adult is unable to see nearby objects. Therefore, it is known as 'Old age hypermetropia'.

 

Normal eye vs. Presbyopia eye
 
Some people have both myopia and hypermetropia, or a combination of the two. 'Bifocal lenses' can help with this. The upper part is made up of a concave lens (to correct myopia) for distance vision, and the lower part is made up of a convex lens (to correct hypermetropia) for reading.
 
Astigmatism:

In this condition, the eye cannot distinguish between parallel and horizontal lines. It can either be inherited or purchased. It is caused by the imperfect structure of the eye lens as a result of cataract formation on the lens, corneal ulceration, and injury to the refracting surfaces, among other things. Cylindrical lenses can be used to correct astigmatism.