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Accommodation is the process by which the eye changes optical focus (from distant to near and vice versa) due to muscular re-shaping of the eye's lens. It can either be automatic or under voluntary control.
The mechanism involves a smooth muscle group called ciliary muscle fibers which surround the eyeball. It was first described in 1855 by Hermann von Helmholtz, who suggested that when viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens 'zonules' (supporting fibres) to slacken - allowing the lens to relax into a thicker, more convex, form.
The widely accepted 'Helmholtz' theory has recently been called into question - particularly due to its failure to take into account changes in lens shape caused by gravity and ageing. In 1970 an alternative mechanism called the 'Catenary' theory was proposed. And in 1992 a third theory called the 'Schachar' was suggested.
The three theories are described in detail at Wikipedia
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