Spherical Abberation occur because lens has a spherical surface and not a parabolic one. Also, rays making a large angles with respect to the optic axis are bought to a different focus.

Show Spherical Abberation with Real Ray Tracing

Since spherical abberation occur because to third order, we cannot use linear approximation to show that spherical abberation exist. Therefore, we use real ray tracing we learn in class to show that the focal point are different depending on where the first hit occur.

Second Example:

We know that the smaller the "f-stop", the larger the aperture from this equation

Therefore, to fix spherical abberation, we have to make the aperture smaller by putting in a stop to limit the ray form large angle from coming in.