Accommodation

Accommodation is the process by which the eye increases optical power to maintain a clear image (focus) on the retina. The principal focusing ability of the (terrestrial) eye is due to the difference in refractive index between and the curved cornea, but the variable curvature of the lens allows for an additional adjustment. This varies from a maximum of over 15 diopters in an infant to only about 1.5 diopters in a person 70 years old, as the lens becomes less flexible with age.

When viewing a far object, the circularly arranged ciliary muscle relaxes causing the lens zonules and suspensory ligaments to pull on the lens, flattening it. Flatter lens has reduced refractive power. When viewing a near object, the ciliary muscles contract causing the lens zonules to slacken which allows the lens to spring back due to elasticity into a thicker, more convex, form. This convex form has increased refractive power.

This theory was introduced by Helmholtz and can be easily verified through the use of atropine, or some other atropine derivate. There was also some other theories that involved the use of extraocular muscles, but these are rejected as there is no scientific proof.

There is a theory that prolonged accommodation can be responsible for axial eye elongation that is responsible for nearsightedness (myopia). There are some scientific studies that do show accommodation can induce a very slight axial elongation. Prolonged accommodation is currently only considered as factor that can worsen the condition, but other factors are necessary. Many people do intense near work for a very prolonged period of time and never develop myopia

Presbyopia is caused by inadequate accommodation. Over time lens loses its elasticity and become rigid, thus unable to change its curvature. People become unable to see clearly near objects and often require reading glasses. To read more about presbyopia, go here.

Aphakia is the absence of the lens of the eye due to surgical removal, perforating wound or ulcer, or congenital anomaly; causes a loss of accommodation, hyperopia, and a deep anterior chamber.

People with aphakia are unable to accommodate and suffer from high degree of hypermetropia.

Unfortunately no. Artificial lens cannot be adequately incorporated in the ciliary apparatus and is unable to change refractive power. People with artificial lens permanently lose accommodation ability.