This paper describes a simplified procedure to determine the modulation transfer function (MTF) of the focal spot of mammographic equipment. To make this quality evaluation method applicable to routine clinical use of mammography units, a different computer method is proposed, which discards most of the experimental apparatus required in the traditional form using slit image digitization. In our method the determination of the MTFs is provided from the corresponding line spread functions in several field directions from focal spot projections, in replacement to the digitization procedure. The MTFs obtained by our procedures are in good agreement compared to those obtained using the conventional experimental techniques described in the literature. This technique is performed on any available PC and it allows us to know also the effect of geometrical unsharpness for all directions in the field of nonisotropic mammography units. This method may make it possible to perform quality control routinely by the transfer function method, considered to be the most complete and accurate method for evaluating radiographic system quality.