Spherical Aberration
An image blur defect in spherical lenses and mirrors where marginal rays focus closer to the element than paraxial central rays.
Governing formula
Δf_spherical = f_marginal - f_paraxial
SI unit
m
In depth
Spherical surfaces are not perfectly parabolic. Marginal rays striking near the outer edges undergo excessive refraction or reflection.
Examples in the real world
Blurry unsharp telescope images corrected using parabolic mirrors or aspheric corrective lens elements.