The various rays in a wide beam falling on a concave mirror of large aperture after reflection don’t meet at a single focus, marginal rays get focused somewhere between F & F while paraxial between C & F , consequently the image formed is not bright & single but a large no. of blurred images are produced.
In order to minimize the spherical aberration defect we shall use
(a) Objects with low heights
(b) Paraxial incident rays
(c) Mirrors of small aperture (called parabolic mirror, used as reflectors of head lamps of vehicles).