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Presbyopia (from Greek ''presbys'' (πρέσβυς) = old man + ''ops'' (ὤψ) = see like, sight > meaning literally ''trying to see as old men do''〔(【引用サイトリンク】title=Presbyopia )〕) is a condition associated with aging in which the eye exhibits a progressively diminished ability to focus on near objects. Presbyopia’s exact mechanisms are not fully understood; research evidence most strongly supports a loss of elasticity of the crystalline lens, although changes in the lens’s curvature from continual growth and loss of power of the ciliary muscles (the muscles that bend and straighten the lens) have also been postulated as its cause. The first signs of presbyopia – eyestrain, difficulty seeing in dim light, problems focusing on small objects and fine print – are usually first noticed between the ages of 40 and 50. The ability to focus on near objects declines throughout life, from an accommodation of about 20 dioptres (ability to focus at 50 mm away) in a child, to 10 dioptres at age 25 (100 mm), and levels off at 0.5 to 1 dioptre at age 60 (ability to focus down to 1–2 meters only). The expected, maximum, and minimum amplitudes of accommodation in diopters (D) for a corrected patient of a given age can be estimated using Hofstetter's formulas: Expected amplitude (D) = 18.5 - 0.3 x (age in years), Maximum amplitude (D) = 25 - 0.4 x (age in years), Minimum amplitude (D) = 15 - 0.25 x (age in years).〔Robert P. Rutstein, Kent M. Daum, ''Anomalies of Binocular Vision: Diagnosis & Management'', Mosby, 1998.〕 ==Signs and symptoms== The first symptoms most people notice are difficulty reading fine print, particularly in low light conditions, eyestrain when reading for long periods, blur at near or momentarily blurred vision when transitioning between viewing distances. Many extreme presbyopes complain that their arms have become "too short" to hold reading material at a comfortable distance.〔Robert Abel, ''The Eye Care Revolution: Prevent and Reverse Common Vision Problems'', Kensington Books, 2004.〕 Presbyopia symptoms, like other focus defects, become much less noticeable in bright sunlight due to the action of the iris closing to a smaller diameter. As with any lens, increasing the focal ratio of the lens increases depth of field by reducing the level of blur of out-of-focus objects (compare the effect of aperture on depth of field in photography). A delayed onset of seeking correction for presbyopia has been found among those with certain professions and those with miotic pupils. In particular, farmers and homemakers seek correction later, whereas service workers and construction workers seek eyesight correction earlier. Scuba divers with interest in underwater photography may notice presbyopic changes while diving before they recognize the symptoms in their normal routines due to the near focus in low light conditions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「presbyopia」の詳細全文を読む スポンサード リンク
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