Spherical aberration

Spherical aberration is an optical defect in which rays passing through different zones of a spherical surface focus at different positions, blurring the image.

Paraxial approximation: An optical approximation that treats rays as close to the optical axis and angles as small. It predicts a common focus for near-axis rays, unlike the behavior of wider-aperture rays.
Refraction: The change in direction of light as it crosses between materials with different refractive indices. Spherical aberration arises because refraction varies across a curved surface.
Aspheric lens: A lens with at least one surface that is not part of a sphere. Its tailored curvature can bring marginal and paraxial rays toward a shared focus.
Coma (optics): An off-axis optical aberration that makes point sources appear comet-shaped. Coma distorts off-axis images, whereas spherical aberration affects focus even on-axis.
René Descartes: A French philosopher and mathematician whose work included a seventeenth-century theory of refraction and lens design. His geometric approach helped frame the problem of focusing rays with curved surfaces.
Marginal ray: A ray that passes near the edge of an optical element or aperture. Marginal rays typically focus closer to a spherical lens than paraxial rays.
Geometric optics: The model of light as rays traveling along paths governed by reflection and refraction. The usual description of spherical aberration follows individual rays through an optical system.
Stopping down: Reducing a camera lens's aperture by closing its diaphragm. A smaller opening excludes marginal rays and often reduces spherical aberration.
Astigmatism (optical systems): An optical aberration in which rays in perpendicular planes focus at different distances. Astigmatism separates tangential and sagittal foci rather than foci by spherical zone.
Christiaan Huygens: A Dutch mathematician and physicist who designed lenses and developed a wave theory of light. His lens designs pursued sharper images by controlling aberrations.

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