In the shape-from-focus method, the position and shape of an object are determined from a series of photographic images obtained at different focal planes in the camera. Each point of the object is focused on a particular focal plane, and the set of all such image points is defined as the focused image surface (FIS). Once the FIS is determined, the original surface can be obtained by a simple projective transformation. Practical methods for determining the FIS have been discussed in many publications, which are listed in this paper. The refinement proposed in this paper consists of an approximation of the FIS by piecewise linear instead of step functions. Some details of the method are given. Experiments show some slight improvement over the older procedures.