Show that this is the case using the simple lens formula for the focal length of a thin lens?

It was stated that a pressure difference across a flat window bows the window into a weak negative lens (i.e., a lens with a negative focal length, and little refractive power). Show that this is the case using the simple lens formula for the focal length of a thin lens? It was stated that a small sphere on a flat surface of infinite radius has a higher stress than the same sphere on a surface that curves away from it. Is the stress higher or lower when the mating surface curves toward the small sphere?