Get to the root of it 
Spherical aberration in lenses
Spherically shaped lenses and mirrors share a problem: their shape. Parallel light rays that pass through the central region of a spherical lens focus farther away than light rays that pass through the edges of the lens. The result is many focal points, which produce a blurry image.
The quest for one and only one focal point
If there were a "perfect" lens, all light rays would be guided through one and only one focal point and the resulting image would be perfectly clear.