Aspherical spectacle lenses use varying curves across their surface to reduce bulk and make them flatter in their profile. Spherical lenses use a singular curve in their profile, making them simpler but bulkier, especially in the center of the lens.
Perhaps the most powerful truism about asphericity is that vision through aspheric lenses is closer to natural vision. Aspheric design allows flatter base curves to be used without compromising optical performance. The basic difference between a spherical and an aspheric lens is that a spherical lens has one curvature and is shaped like a basketball. An aspheric lens curves gradually, like the football below. The aspheric lens reduces magnification to make the appearance more natural and the decreased center thickness uses less material, resulting in less weight.