Spherical aberration is an optical flaw inherent in lenses with standard spherical surfaces, causing marginal rays to refract more strongly than paraxial rays. This results in a blurred, spread-out focal spot instead of a sharp point, lowering the image contrast and resolution. Designers correct it by combining positive and negative lenses, using aspheric surfaces, or stopping down the aperture to block the edge rays.
The beam profile looks terrible at the focus because that cheap plano-convex lens is introducing massive spherical aberration.
A quick lab fix for spherical aberration is flipping a plano-convex lens so its curved side faces the collimated beam, which minimizes the refraction angles at the surfaces.
Related terms
Longitudinal aberration
The distance along the optical axis between the focal points of paraxial rays and marginal rays.
Best form lens
A single spherical lens designed with specific surface curvatures to minimize spherical aberration.
Aplanat
A lens designed to be free of both spherical aberration and coma.
Aperture
A physical opening that limits the bundle of light rays passing through a system.