Presbyopia is a physiological insufficiency of optical accommodation associated with the aging of the human eye; it results in progressively worsening ability to focus clearly on close objects.
Presbyopia is a typical part of the aging process. It occurs due to age-related changes in the lens (decreased elasticity and increased hardness) and ciliary muscle (decreased strength and ability to move the lens), causing the eye to focus right behind rather than on the retina when looking at close objects. It is a type of refractive error, along with nearsightedness, farsightedness, and astigmatism. Diagnosis is by an eye examination.
Presbyopia can be corrected using glasses, , multifocal intraocular lenses, or LASIK (PresbyLASIK) surgery. The most common treatment is glass correction using appropriate convex lens. Glasses prescribed to correct presbyopia may be simple reading glasses, bifocals, trifocals, or .
People over 40 are at risk for developing presbyopia and all people become affected to some degree. An estimated 25% of people (1.8 billion globally) had presbyopia .
Presbyopia, like other focal imperfections, becomes less noticeable in bright sunlight when the pupil becomes smaller. As with any lens, increasing the F-number 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).
The onset of presbyopia varies among those with certain professions and those with miotic . In particular, farmers and homemakers seek correction later, whereas service workers and construction workers seek 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.
A surgical technique offered is to create a "reading eye" and a "distance vision eye", a technique commonly used in contact lens practice, known as monovision. Monovision can be created with contact lenses, so candidates for this procedure can determine if they are prepared to have their corneas reshaped by surgery to cause this effect permanently.
In optics, the closest point at which an object can be brought into focus by the eye is called the eye's near point. A standard near point distance of is typically assumed in the design of optical instruments, and in characterizing optical devices such as .
There is some confusion over how the focusing mechanism of the eye works. In the 1977 book, Eye and Brain,
for example, the lens is said to be suspended by a membrane, the 'zonula', which holds it under tension. The tension is released, by contraction of the ciliary muscle, to allow the lens to become more round, for close vision. This implies the ciliary muscle, which is outside the zonula, must be circumferential, contracting like a sphincter, to slacken the tension of the zonula pulling outwards on the lens. This is consistent with the fact that our eyes seem to be in the 'relaxed' state when focusing at infinity, and also explains why no amount of effort seems to enable a myopic person to see farther away.The ability to focus on near objects declines throughout life, from an accommodation of about 20 (ability to focus at away) in a child, to 10 dioptres at age 25 (), and levels off at 0.5 to 1 dioptre at age 60 (ability to focus down to 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 (age in years); maximum amplitude (age in years); minimum amplitude (age in years).Robert P. Rutstein, Kent M. Daum, Anomalies of Binocular Vision: Diagnosis & Management, Mosby, 1998.
A basic eye exam, which includes a refraction assessment and an eye health exam, is used to diagnose presbyopia.
Contact lenses can also be used to correct the focusing loss that comes along with presbyopia. Multifocal contact lenses can be used to correct vision for both the near and the far. Some people choose contact lenses to correct one eye for near and one eye for far with a method called monovision.
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