Presbyopia is a vision condition where the eye progressively losses its ability to focus (eye accommodation) on nearer objects with age.
Presbyopia may develop due to loss of elasticity of the crystalline lens. Other contributing factors are changes in the lens curvature and loss of strength of the ciliary muscles (the muscles that change lens curvature by bending and straightening the lens).
Graying of hair and development of wrinkles are considering as normal with aging, similarly presbyopia is also considering as the natural course of aging.
Accommodation is the process in which the eye increases its optical power (the degree to which the lens converges or diverges from the light) by modifying the curvature of the eye lens with the help of ciliary muscle. This is important to produce a clear image of an object that is closer to the eye.
Eye accommodation is a physiological act of adjusting crystalline lens elements to alter the refractive power and bring objects that are nearer to the eye into clear sharp focus.
To focus on distant objects the ciliary muscles relax and make the eye lens thin. This increases the focal length of the eye lens and thus distance objects are seeing clearly. However, to focus on nearby objects the ciliary muscles contracts and make the eye lens thick. This decreases the focal length of the eye and which helps to see nearer objects clearly.
Aging makes the ciliary muscles losses in original strength as well as the crystalline lens losses its elasticity thus accommodation becomes difficult, which end up in presbyopia (an age-related vision problem).
Eye accommodation decreases with age
Eye accommodation is good when the age is below 40 years, during this period the eye can compare with an auto-focus camera (gets sharp pictures with varying object distance). After age 40, eye accommodation, mechanism looses its ability and develops presbyopic eye, which can compare with a fixed-focus camera (not able to get sharp pictures with varying object distance).