Accommodation (eye)

Accommodation (eye)

Accommodation (Acc) is the process by which the vertebrate eye changes optical power to maintain a clear image (focus) on an object as its distance varies.

Accommodation acts like a reflex, but can also be consciously controlled. Mammals, birds and reptiles vary the optical power by changing the form of the elastic lens using the ciliary body (in humans up to 15 diopters). Fish and amphibians vary the power by changing the distance between a rigid lens and the retina with muscles.

The young human eye can change focus from distance (infinity) to 7 cm from the eye in 350 milliseconds. This dramatic change in focal power of the eye of approximately 13 diopters (diopter is the reciprocal of focal length in meters) occurs as a consequence of a reduction 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 to less than 2 dioptres by the time a person reaches 45 to 50 years, by which time most of the population will have noticed a decrease in their ability to focus on close objects and hence require glasses for reading or bifocal lenses. Accommodation decreases to essentially 0 dioptres at the age of 70 years.

It is normally accompanied by a convergence of the eyes to keep them directed at the same point, sometimes termed accommodation convergence reflex.