Well...as some one who has relied on prescription glasses and other types of corrective lenses for the majority of her life....many people need glasses because for some reason the curvature of the eye...i.e. the curvature of our "lens" is not as "optimal" for certain distances considered normative in human vision (sometimes very small distances or very large ones). meaning that due to too little or too much lens curvature the ability to curve light appropriately so that a point light source converges fully at a photoreceptor is somehow off (i.e. the convergence of the bent light does not occur where it is supposed to) thus, images becomes increasingly blurred. Also, instances where astigmatism is present indicate a situation where an aberration or other topographical situation has arisen in which different parts of the lens and cornea focus and bend light rays differently (i.e. the curvature and focusing ability of the optical components of the eye are not necessarily uniform) creating two different areas (versus one) of sharp focus in one's vision (i.e. the eye focuses on two different points in space at once). Of course there are many other reasons why an individual may lose eye-sight some of them are congenital conditions (probably stemming from genetic makeup) while others may be influences or caused by life experience and/or environmental factors which may affect other parts of they eye. However, one of the most interesting things, for me, is how plastic eye-sight can be and how eye-sight is so quick to change at times. For instance, while I know some people with degenerative eye conditions in which they slowing lose eye-sight, I also know other people, such as my cousin, who (albeit with the help of eye-surgery) have obtained near 20-20 vision after being nearly legally blind for the majority of their childhood.