Today almost all of the IOLs that we use have an aspheric design with either negative spherical aberration or zero spherical aberration. This video was made from a powerpoint presentation that I created in 2006! And it will give you a concise summary to understand aspheric IOLs.
Spherical lenses have a constant curvature on their surface, much in the same way that a sphere has a constant curvature. However, the power of the lens at different points is variable, resulting in relative defocus of more peripheral light rays. Due to this constant curvature, these lenses are easier to make; however, their optics are worse. A perfectly aspheric lens has a variable curvature but a constant power even at the periphery of the lens. This results in the same focus of all light rays, both central and peripheral. These lenses are more challenging to make, but their optics are better.
Peripheral light rays are relatively defocused in spherical lenses, resulting in a degradation of image quality. In an aspheric lens, the peripheral light rays are focused at the same point as the central rays, resulting in a sharper, higher-quality image.
Because of their superior optics, aspheric lenses have become the standard for high-end cameras, surgical microscopes, slit lamps and even high-quality glasses and contact lenses. In fact, for nearly 50 years ophthalmologists have been using aspheric 20 D lenses to examine retinas via the binocular indirect ophthalmoscope. The benefits of the aspheric 20 D lens are many: the image quality is good, there is no distortion of the image at the periphery of the lens and the lens is relatively immune to decentration or tilt. For comparison, try examining the retina with the 20 D from your spectacle trial lens set!