http://en.wikipedia.org/wiki/Eye
A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina.This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. Such an in homogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.
A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges; this decreases the focal length and thus allows a sharp image to form on the retina.This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration. Such an in homogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.
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