Spherical Aberration

All incoming light rays parallel to the principal axis of a parabolic mirror pass through the focal point. For a spherical mirror this is only approximately true. The error for spherical mirrors increases as the distance from the principal axis grows. The inability of spherical mirrors to reflect all incoming parallel rays through the focal point is known as spherical aberration. Ignoring spherical aberration when calculating the paths of rays is known as the paraxial approximation.