Aperture of a lens is the effective diameter of its light transmitting area.
It gives the size of the mirror.
Therefore, the brightness i.e. the intensity of the image formed by a lens which depends on the amount of light passing through the lens will vary as the square of the aperture of the lens.
Mirrors whose aperture is much smaller than its radius of curvature, we use R=2f.
The focal length will remain the same as it does not depend on the aperture of the lens.