Presbyopia ("old-age sight")

 As the lens matures, new fibers are constantly moved to the lens cortex, while the older fibers are enclosed in the lens nucleus. With time this lens hardens due to increase in cortical fibers, resulting in a decrease in the accommodative response to the contraction of the ciliary muscle.
What is presbyopia?
Presbyopia is a condition associated with aging of the eye that results in progressively worsening ability to focus clearly on near objects.
Presbyopia is a natural part of the aging process. Anyone over the age of 35 is at risk of having presbyopia and all the people become affected to some degree. Once presbyopia 0ccurs, it gradually increases over a period of 10 and 12 years and then stabilizes. A presbyope typically requires +1.00 D reading addition in the early 40s, leveling off at about a +2.25 or +2.50 D addition by about the age of 55. However, once presbyopia is evident, many emmetropes or low hyperopes undergo an increase in hyperopia after reading glasses have been worn for several months. For example, a 43-year-old emmetrope requiring +1.00 D reading lenses may find that by the age of 45 the +1.00 D lenses are no longer sufficient for close work but are just right for distance vision. This is sometimes referred to as the “hyperopia of presbyopia”
Presbyopia occurs when the near point of accommodation has receded to the point that it is difficult or impossible to accommodate sufficiently for reading or other close work.
For most people close work becomes difficult when the amplitude of accommodation is less than 5.00 D (corresponding to the near point of accommodation-while wearing lenses to correct any distance refractive error-of 20cm0. Presbyopia is relieved by convex lenses for near work, either in form of reading glasses or bifocal lenses
Symptoms include a hard time reading small print, having hold reading materials farther away, blurred vision, headaches, and eyestrain.
An old rule of thumb is that a person will find close work to be comfortable if no more than half of the amplitude of accommodation must be used. If it is assumed that close work is done at distance of 40cm, requiring 2.50 D of accommodation, this rule of thumb tells us that a person with 5.00 D of accommodation would require o reading glasses or bifocal; however, a person with 3.00 D of accommodation should have to routinely use 1.50 D of accommodation and, therefore, should have a 1.00 D addition in the form of reading lenses or bifocal.