English: Figure illustrates use of positive & negative curvatures for the surfaces of a lens.

Think of the principle axis as being a real number line, with the mid-plane marking the origin of the axis. Light rays travel in the positive direction.

IF the center of curvature – for either surface – lies in the negative region, then the curvature is negative, R < 0.

IF the center of curvature – for either surface – lies in the positive region, then the curvature is positive, R > 0.