The principal focus is defined as the point where a beam parallel to the principal axis appears to diverge converges from a point on the principal axis after passing through the lens.
Principal focus of Convex Mirror
A number of rays which falls parallel to the principal axis after getting reflected, the rays seems to come from a point on the principal axis. This is called as Principal focus Convex Mirror.
Principal focus of Concave Mirror
A number of rays which falls parallel to the principal axis on a concave mirror meets at a point on the principal axis after getting reflected. This is known as the principal focus of a concave mirror.
The incident rays are converged by a convex lens. As a result, the rays converge in front of the lens at a single point. As a result, the image distance is positive in this case. Image distance is negative in the case of concave leans.