The Cornea and Lens:

  The cornea is a positive meniscus lens.  It is less curved at its periphery, reducing spherical aberration (a blurring of the image).

  The lens is a positive biconvex lens, with two surfaces of unequal curvature.

  The index of refraction of the lens is lower at the surface and higher in the center, also reducing spherical aberration (the biological equivalent of a very modern GRIN lens).

  The lens is made up of layers of transparent fiber, like an onion.  These layers are pliable in our younger years, allowing the lens to change shape and focus when looking at objects up close or far away (accommodation).  As we age, these layers become harder and less elastic, reducing the range over which we can accommodate.  As a result, we need to wear glasses or contact lenses.