The image irradiance equation constrains the relationship between surface orientation in a scene and the irradiance of its image. This equation requires detailed knowledge of both the scene illumination and the reflectance of the surface material. For this equation to be used to recover surface orientation from image irradiance, additional constraints are necessary. The constraints usually employed require that the recovered surface be smooth. We demonstrate that smoothness is not sufficient for this task. A new formulation of shape from shading is presented in which surface orientation is related to image irradiance without requiring detailed knowledge of the scene illumination, or of the albedo of the surface material. This formulation, which assumes isotropic scattering, provides some interesting performance parallels to those exhibited by the human visual system.