Spherical

EN: Spherical aberration

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This aberration exists to some degree in all lenses constructed entirely of spherical elements. Spherical aberration causes parallel light rays passing through the edge of a lens to converge at a focal point closer to the lens than light rays passing through the center of the lens. (The amount of focal point shift along the optical axis is called longitudinal spherical aberration.) The degree of spherical aberration tends to be larger in large-aperture lenses. A point image affected by spherical aberration is sharply formed by light rays near the optical axis but is affected by flare from the peripheral light rays (this flare is also called halo, and its radius is called lateral spherical aberration). As a result, spherical aberration affects the entire image area from the center to the edges, and produces a soft, low-contrast image which looks as if covered with a thin veil. Correction of spherical aberration in spherical lenses is very difficult. Although commonly carried out by coming two lenses-one convex and one concave-based on light rays with a certain height of incidence (distance from the optical axis), there is a limit to the degree of correction possible using spherical lenses, so some aberration always remains. This remaining aberration can be largely eliminated by stopping down the diaphragm to cut the amount of peripheral light. With large aperture lenses at full aperture, the only effective way to thoroughly compensate spherical aberration is to use an aspherical lens element.
Canon glossary EU

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Spherical

AKA: Spherical PrintAn optical system in which the magnifications of the vertical and horizontal dimensions of the picture are the same. See also aspect ratio, contrast with anamorphic.

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Spherical aberration

One of Seidel s five aberrations. It is a variation in the focal length of a lens from the center to the edges, due to its shape. Spherical aberration appears as a halo, blur and loss of sharpness. It becomes more common at wide apertures, and can be reduced by stopping down the lens. It can also be reduced by the optimal combination of positive and negative lenses.

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