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Accommodation is the process by which the eye 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.
Helmholtz - The most widely held theory of accommodation is that proposed by Hermann von Helmholtz in 1855. When focusing at near the circular muscle fibers of the ciliary muscle contract decreasing the equatorial circumlenticular space which reduces zonular tension and allows the lens to round up and increase in optical power lens zonules. When viewing a distance object the circular ciliary muscle fibers relax which increases the equatorial circumlenticular space causing an increase in zonular tension. The increase in zonular tension causes the surfaces of the lens to flatten and the optical power of the lens to decrease. Helmholtz’s theory of accommodation is inconsistent with the well-documented flattening of the anterior peripheral surfaces of the lens and negative shift of spherical aberration that occurs during human in vivo accommodation.
Schachar - Ronald Schachar has contributed scientific insight into the mechanism of human accommodation, indicating that focus by the human lens is associated with increased tension on the lens via the equatorial zonules. Moreover, the evidence supporting the Schachar hypothesis disproves the older theory concerning the mechanism of accommodation of von Helmholtz. Schachar found that when the ciliary muscle contracts, equatorial zonular tension is increased. The increase in equatorial zonular tension causes the central surfaces of the crystalline lens to steepen, the central thickness of the lens to increase (anterior-posterior diameter), and the peripheral surfaces of the lens to flatten. While the tension on equatorial zonules is increased during accommodation, the anterior and posterior zonules are simultaneously relaxing.  As a consequence of the changes in lens shape during human in vivo accommodation, the central optical power of the lens increases and spherical aberration of the lens shifts in the negative direction.  Because of the increased equatorial zonular tension on the lens during accommodation, the stress on the lens capsule is increased and the lens remains stable and unaffected by gravity.  The same shape changes that occur to the crystalline lens during accommodation are observed when equatorial tension is applied to any encapsulated biconvex object that encloses a minimally compressible material (volume change less than approximately 3%) and has an elliptical profile with an aspect ratio ≤ 0.6 (minor axis/major axis ratio).  Equatorial tension is very efficient when applied to biconvex objects that have a profile with an aspect ratio ≤ 0.6. Minimal equatorial tension and only a small increase in equatorial diameter causes a large increase in central curvature. This explains why the aspect ratio of a vertebrate crystalline lens can be used to predict the qualitative amplitude of accommodation of the vertebrate eye. Vertebrates that have lenses with aspect ratios ≤ 0.6 have high amplitudes of accommodation; e.g., primates and falcons, while those vertebrates with lenticular aspect ratios > 0.6 have low amplitudes of accommodation; e.g. owls and antelopes.  The decline in the amplitude of accommodation eventually results in the clinical manifestation of presbyopia; i.e., when the near focal point of the eye is more remote than the near reading distance. It has been widely suggested that the age-related decline in accommodation that leads to presbyopia occurs as a consequence of sclerosis (hardening) of the lens. However, the lens does not become sclerotic until after 40 years of age. In fact, the greatest decline in the amplitude of accommodation occurs during childhood, prior to the time that any change in hardness of the lens has been found. The decline in accommodative amplitude, rapid in childhood and slow thereafter, follows a logarithmic pattern that is similar to that of the increase in the equatorial diameter of the lens, which is the most likely basis for the accommodative loss.  As the equatorial diameter of the lens continuously increases over life, baseline zonular tension simultaneously declines. This results in a reduction in baseline ciliary muscle length that is associated with both lens growth and increasing age. Since the ciliary muscle, like all muscles, has a length-tension relationship, the maximum force the ciliary muscle can apply decreases, as its length shortens with increasing age. This is the etiology of the age-related decline in accommodative amplitude that results in presbyopia.  Any procedure that can prevent equatorial lens growth or increase the effective distance between the lens equator and the ciliary muscle can potentially increase the amplitude of accommodation. 
Catenary - D. Jackson Coleman proposes that the lens, zonule and anterior vitreous comprise a diaphragm between the anterior and vitreous chambers of the eye.  Ciliary muscle contraction initiates a pressure gradient between the vitreous and aqueous compartments that support the anterior lens shape in the mechanically reproducible state of a steep radius of curvature in the center of the lens with slight flattening of the peripheral anterior lens, i.e. the shape, in cross section, of a catenary. The anterior capsule and the zonule form a trampoline shape or hammock shaped surface that is totally reproducible depending on the circular dimensions, i.e. the diameter of the ciliary body (Müeller’s muscle). The ciliary body thus directs the shape like the pylons of a suspension bridge, but does not need to support an equatorial traction force to flatten the lens.
Duke-Elder classified a number of accommodative dysfunctions:
Paralysis of accommodation
Spasm of accommodation
^ Schachar RA. The mechanism of accommodation and presbyopia. International Ophthalmology Clinics. 46(3): 39-61, 2006
^ Abolmaali A, Schachar RA, Le T. “Sensitivity study of human crystalline lens accommodation.” Computer Methods and Programs in Biomedicine. 85(1): 77-90, 2007
^ Schachar RA, Davila C, Pierscionek BK, Chen W, Ward WW. The effect of human in vivo accommodation on crystalline lens stability. British Journal of Ophthalmology. 91(6): 790-793, 2007.
^ Schachar RA. The lens is stable during accommodation.. Ophthalmic Physiological Optics. In press, 2007.
^ Schachar RA, Fygenson DK. Topographical changes of biconvex objects during equatorial traction: An analogy for accommodation of the human lens. British Journal of Ophthalmology. In press, 2007.
^ Schachar RA, Pierscionek BK, Abolmaali A, Le, T. The relationship between accommodative amplitude and the ratio of central lens thickness to its equatorial diameter in vertebrate eyes. British Journal of Ophthalmology. 91(6): 812-817, 2007.
^ Schachar RA. Equatorial lens growth predicts the age-related decline in accommodative amplitude that results in presbyopia and the increase in intraocular pressure that occurs with age. International Ophthalmology Clinics. 48(1): In press, 2008.
^ Schachar RA, Abolmaali A, Le T. Insights into the etiology of the age related decline in the amplitude of accommodation using a nonlinear finite element model of the accommodating human lens. British Journal of Ophthalmology. 90: 1304-1309, 2006.
^ Schachar RA. The mechanism of accommodation and presbyopia. International Ophthalmology Clinics. 46(3): 39-61, 2006.
^ Coleman DJ. Unified model for the accommodative mechanism. Am J Ophthalmol 1970, 69:1063-79.
^ Coleman DJ. On the hydraulic suspension theory of accommodation. Trans Am Ophthalmol Soc 1986, 84:846-68.
^ Coleman DJ, Fish SK. Presbyopia, Accommodation, and the Mature Catenary. Ophthalmol 2001; 108(9):1544-51.
^ Duke-Elder, Sir Stewart (1969). The Practice of Refraction (8th ed.). St. Louis: The C.V. Mosby Company. ISBN 0-7000-1410-1.