As evidenced by many physiological and psychophysical reports, the receptive fields of the first-stage set of mechanisms of the visual process fit to two-dimensional (2D) compactly supported harmonic functions. The application of this set of band-pass filter functions to the input signal implies that the visual system carries out some kind of conjoint space/spatial frequency transform. Assuming that a conjoint transform is carried out, we present in this paper a new characterization of the visual system performance by means of a weighting function in the conjoint domain. We have called this weighting function (in the particular case of the Gabor transform) the Gabor stimuli Sensitivity Function (GSF) by analogy with the usually employed weighting function in the Fourier domain: the Contrast Sensitivity Function (CSF). An analytic procedure to obtain this conjoint weighting function from the psychophysical measurements of the CSF is derived. The accuracy of the procedure is proved showing the equivalence between some experimental Fourier weighting functions (2D CSFs), and the corresponding GSFs. The main advantage of this new characterization is that a weighting function in a space/spatial frequency domain can account for spatially variant behaviour, which cannot be included in a unique CSF, so a single GSF would be needed to include extra-foveal and large eccentricity behaviour. One example is given of how non-homogeneous systems can be easily characterized in this way.