I've tested your last setup with a single sided square and a very thin box prop. Changing the render settings had no effect. Playing with the material settings had no effect (even when "reflection + refraction = 1"). Changing any of the colors in the materials had no effect. There was nodifference as long as the single sided square was in the SAME distance to the face as the front side of the box prop. This distance between the front of the glass and the surface is the only crucial parameter. So far - the only way to prevent those ugly artefacts is to increase the distance between the glasses and the face. Make the lenses smaller so you can have a greater distance to the side of the nose. This seems to be a real flaw in the firefly renderer. Willy