Correcting the spherical aberration
In spherical lenses peripheral rays are more strongly refracted than rays near the optical axis. This aberration is known as spherical aberration.
The spectacle lens wearer will perceive this aberration as blur, especially when not looking through the optical centre of the lens.
The solution: Aspherical Surface Design
If the surface of the lens is "folded open" in the peripheral areas so that the surface structure deviates from the spherical shape (aspheric surface). All rays coming from the distance meet again at one point. The aberration is corrected.
A positive side-effect of this flattening is that it leads to thinner and lighter lenses. This effect is most evident with high plus powers. In this case, the reduction of the centre thickness also leads to a reduced magnifying effect (eyes look more natural).