When the surfaces of a lens are pieces of spheres, the center of the lens focuses an image farther from the lens than the edges. (The edges of a spherical lens are too sloped.) this smears out the focus and is called SPHERICAL ABERRATION.

Water Sphere Lens

Aberration - Astigmatism
Aberration - Barrel
Aberration - Chromatic
Aberration - Coma
Aberration - Curvature of Field
Aberration - Pincushion

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