Spherical aberration
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Spherical-aberration-disk.jpg
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The effect is proportional to the fourth power of the diameter and inversely proportional to the third power of the focal length, so it's much more pronounced at short focal ratios, i.e. fast lenses.
For small telescopes using spherical mirrors with focal ratios shorter than f/10 light from a distant point source (such as a star) is not all focused at the same point. Particularly, light striking the inner part of the mirror focuses further from the mirror than light striking the outer part of the mirror. As a result the image cannot be focused as sharply as if the aberration were not present. Because of spherical aberration telescopes shorter than f/10 are usually made with non-spherical mirrors or with correcting lenses.
In lens systems, the effect can be minimized using special combinations of convex and concave lenses, as well as using aspherical lenses.