Explain power of accomodation
also explain what happens when ciliary muscles contract and relax (does the focal length increase or decrease) (what happens to the reading power if it increases or decreases)
also explain what happens when ciliary muscles contract and relax (does the focal length increase or decrease) (what happens to the reading power if it increases or decreases)
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Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies.
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Accommodation usually acts like a reflex, including as part of the accommodation-vergence reflex, but it can also be consciously controlled.
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The young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye.This dramatic change in focal power of the eye of approximately 15 dioptres (dioptre is the reciprocal of focal length in metres) occurs as a consequence of a reduction in tension induced by ciliary muscle contraction.
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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 about 1 dioptre at the age of 70 years.
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The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude can decline so that 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
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Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies.
-
Accommodation usually acts like a reflex, including as part of the accommodation-vergence reflex, but it can also be consciously controlled.
-
The young human eye can change focus from distance (infinity) to as near as 6.5 cm from the eye.This dramatic change in focal power of the eye of approximately 15 dioptres (dioptre is the reciprocal of focal length in metres) occurs as a consequence of a reduction in tension induced by ciliary muscle contraction.
-
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 about 1 dioptre at the age of 70 years.
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The amplitude of accommodation declines with age. By the fifth decade of life the accommodative amplitude can decline so that 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
