That the spherical aberration of a converging lens is corrected by reducing the thickness of the lens periphery means that the light rays emerging from aberrant converging lenses:

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11ecba2d_131f_d325_a3bf_0db831587e5d_MD0008_00 An ideal lens generates an image of an object at a single focal point, regardless of the locations at which the light rays enter and exit the lens. Spherical aberration describes the phenomenon by which real lenses' perfectly rounded (ie, spherical) surfaces do not produce an image at a single point, but rather at a series of focal points. In converging lenses, the location of each focal point is related to the distance from the principal axis (ie, the lens center) at which light enters and exits the lens. Spherical aberration is most pronounced among rays entering and exiting the lens periphery (ie, distant from the principal axis). Furthermore, correcting spherical aberration in converging lenses requires using an aspherical lens in which the thickness of the lens periphery is decreased relative to a perfectly rounded lens. Because reducing the thickness of the lens periphery will lead to less refraction of light, light rays exiting the periphery of a spherical converging lens can be said to refract excessively, converging on a focal point that is too close to the lens. (Choice A) Spherical aberration in a converging lens is characterized by refractive events that focus light emerging at the lens periphery more dramatically than light emerging closer to the lens center. Therefore, light is excessively, not insufficiently, refracted at the lens periphery. (Choice B) All lens and mirror aberrations describe a failure of optical instruments to converge or diverge light rays uniformly. (Choice D) Chromatic dispersion describes the failure of a lens to focus multicolored light onto a single point. Chromatic dispersion in converging lenses is corrected by using a diverging lens to increase the thickness of the lens periphery. Educational objective: Spherical aberration occurs when lenses with perfectly rounded surfaces focus light at multiple focal points. Spherical aberration is most pronounced among light rays entering and exiting the periphery of converging lenses.

That the Spherical Aberration of a Converging Lens Is Corrected