Accommodation (eye)

Accommodation is the process by which the increases optical power to maintain a clear image (focus) on an object as it draws near the eye. The young human eye can change focus from distance to 7 cm from the eye in 350 milliseconds. This dramatic change in focal power of the eye of approximately 15 diopters (a diopter is 1 divided by the focal length in meters) occurs as a consequence of a change in zonular tension induced by ciliary muscle contraction. The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude has declined so the near point of the eye is more remote than the reading distance. When this occurs the patient is presbyopic. Once presbyopia occurs, those who are emmetropic (do not require optical correction for distance vision) will need an optical aid for near vision; those who are myopic (nearsighted and require an optical correction for distance vision), will find that they see better at near without their distance correction; and those who are hyperopic (farsighted) will find that they may need a correction for both distance and near vision. The age-related decline in accommodation occurs almost universally, and by 60 years of age, most of the population will have noticed a decrease in their ability to focus on close objects.
See more at Wikipedia.org...