A hardness scale from 0 - 100 used to express the deformability of elastic materials (rubbers). Commonly available O-rings have a hardness of around 70sh. The O-rings used in Nikonos cameras are 60sh. Using too soft an O-ring may result in failure of the seal by extrusion (pushing-out). Using too hard an O-ring will result in extreme stiffness, and potential failure due to excessive wear.