Spherical aberration which affects optical systems performance:
Spherical aberration occurs, “If all the reflected (or) refracted rays doesn’t pass through center of focus”. In other words it can be said as failure of reflected (or) refracted rays that are not passing through center of focus. Spherical aberration is the first term in Seidel aberrations and it is a form of wavefront aberration.
Optical system with spherical aberration:
Spot diagram is the best way to find spherical aberration. It is usual to assume that the light intensity in an image is obtained according to geometrical approximation. But this assumption is completely false when the ray intersection density is infinity.
So, each and every individual ray intersection in the image plane is plotted as isolated points which gives the ray intersection density in image plane and it is called as spot diagram.
Spherical aberration free optical system:
The below figure shows how a aberration free optical system will look like. But in general, it is very difficult to design an aberration free optical system.
 James C Wyant, Katherine Creath, Applied Optics and Optical Engineering 1992, pp. 213-214.
 Born M and Wolf E, Principles of Optic 1959, pp.