Longitudinal aberration quantifies how much a lens fails to bring all parallel rays to a single focal point along the axis of propagation. Rays striking the edge of a spherical lens refract more strongly and focus closer to the lens than rays passing near the center. Optical designers correct this by combining glasses with different refractive indices or using aspheric surfaces to collapse the focal spread.
The spot size is terrible because the longitudinal aberration stretches the focal region out by almost two millimeters.
Related terms
Spherical aberration
An optical defect where light rays passing through the edge of a lens focus at a different point than central rays.
Achromat
An optical assembly designed to minimize chromatic aberration.
Best form lens
A single spherical lens designed with specific surface curvatures to minimize spherical aberration.