A computational model of human cones for intensities ranging from 1 td up to full bleaching levels is presented. The model conforms well with measurements made in primate horizontal cells, follows Weber's law at high intensities, and performs range compression consistent with what is known of cones in other vertebrates. The model consists entirely of processes with a clear physiological interpretation: pigment bleaching, saturation of cGMP hydrolysis, calcium feedback on cGMP synthesis, and a nonlinear membrane. The model is implemented according to a very fast computational scheme useful for simulations, and sample programs in Matlab and Fortran are provided as supplementary material.