It seems that the reflected ray don’t converge at a point but are distributed in a region. For a concave mirror only those parallel rays which are incident on the mirror near the vertex are focussed at ‘F’, other reflected rays form a fuzzy image.
Thus ‘the failure of rays to converge or diverge from a single point after reflection from a spherical mirror is called spherical aberration. ’ This defects destroy the sharpness of the image.
It can be reduced by:
(a) Using mirrors of small aperture.
(b) Putting a stop in front of marginal rays.
(c) Using parabolic mirror instead of spherical mirrors. [It is for this reason that parabolic mirror do not show spherical aberration. Mirrors used in search lights or head lights in automobiles are parabolic]