Canon EOS - Aspherical lenses

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Spherical lenses possess an inherent flaw, in that it's impossible to converge parallel light completely onto a single point. A spherical lens has a rounded surface, which makes light passing through the edges of the lens gather at a point closer to the center of the lens compared with light passing through the center of the lens. The resulting image is slightly blurred; a problem we call "spherical aberration." To correct this spherical aberration, multiple lenses had to be combined in layers. The aspherical lens was developed as a one-piece solution for eliminating spherical aberration.

As the term suggests, the surface of the aspherical lens is "not spherical." Its curvature is slightly changed in the radial direction, and if you look at a cross-section of the lens, you'll find some depression in the lens rim. This shape enables light passing through the lens to converge onto one point, thereby producing a sharp, clear image.

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