What is Spherical Aberration?

Spherical aberration is the monochromatic aberration of an on-axis point and is due to the spherical surface of the lens. The consequence of spherical aberration is that after a point is imaged, it is no longer a bright spot, but a bright spot with the middle bright edge gradually blurred, thus affecting the imaging quality.
  Correction of spherical aberration can be eliminated by cementing concave-convex lenses of different materials.