A convex mirror is a curved mirror in which the reflecting surface bulges towards the light source. Convex mirrors reflect light outwards; therefore they are not used to focus light. A convex mirror is also known as fish eye mirror or diverging mirror. The image formed by a convex lens is virtual and erect, since the focal point (F) and the centre of curvature (2F) are both imaginary points “inside” the mirror that cannot be reached. As a result, images formed by these mirrors cannot be projected on a screen, since the image is inside the mirror. Therefore, its focal length cannot be determined directly. The image is smaller than the object, but gets larger as the object approaches the mirror. The focal length of a convex mirror can be determined by introducing a convex lens between the object and the convex mirror.
This video explains how to find the focal length of a convex mirror using a convex lens.