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Infant eyes: optics and accommodation.

Infant eyes grow rapidly in the first year of life. They exhibit considerable amounts of astigmatism, most frequently in the horizontal/vertical meridians with the horizontal meridian focused myopically relative to the vertical (i.e. 'against-the-rule"). By photokeratometry it is shown that most of this astigmatism in young infants is corneal in nature. The ability to accurately focus a visual target at varying distances exists to some extent from birth, but improves rapidly in the first 4 to 6 months of life until the net spherical error of focus is less than 0.5 diopters for targets 1.5 meters distant. At this stage, the astigmatism of the eye accounts for more than half of the defocus of the worst focused meridian. The persistence of astigmatism beyond the development of accurate focusing raises questions as to its conceivable functional significance. Because the mechanism of accommodation in adults may involve one or more of several methods for detecting the sign of defocus, including astigmatic blur, a possible role of astigmatism in infant focusing mechanisms is described.