Spherical mirrors have an aberration.
The defect is most noticeable for light striking the outer edges of the mirror. Rays which strike the outer edges of the mirror fail to focus in the same precise location as light rays which strike the inner portions of the mirror. While light striking the interior of a mirror may focus to a point, any light striking the edges fails to focus at that same point. The result is that the images of objects as seen in spherical mirrors are often blurry. The diagram below shows six incident rays traveling parallel to the principal axis and reflecting off a concave mirror.