We propose a new algorithm to obtain the color matching functions of any digital camera, with correct weights and white balance, from the relative scaling of their complete (3-D) spectral sensitivities obtained from real spectroradiometric data. Thanks to this algorithm, it is possible to predict the RGB digital levels of any digital camera in realistic illumination-scene environments—spatially non-uniform illumination field, variable chromaticity and large dynamic range of luminance levels—opening the possibility of transforming any digital camera into a tele-colorimeter. The illumination-scene test was the Macbeth Color-Checker Chart under three different light sources (halogen, metal halide and daylight fluorescent lamps), provided by a non-standard light box. The results confirmed that it is possible to predict any RGB digital levels exclusively by varying the f-number of the camera zoom lens.