From real spectroradiometric data, we propose an algorithm to obtain camera opto-electronic conversion functions (camera OECFs) for different values of the lens aperture N from the opto-electronic conversion spectral functions (OECSFs). Working in the linear range of the inverse OECFs, we have found that the slope vs. lens aperture function is fitted by a second order polynomial. This happens also to the offset vs. lens aperture function. Therefore, we have included the inverse OECFs with luminance adaptation into a basic reproduction model, consisting in a 3×3 transform between the raw RGB space and CIE-XYZ space, opening the possibility of transforming any digital camera into an absolute tele-colorimeter.