A new, to the best of our knowledge, way of tuning the focal length of a thermal lens is presented. The thermal lens is formed in a photochromic thermo-optical material, through illumination of the material by a heating laser beam of constant power. The tunability of the lens is achieved by changing the absorption coefficient of the thermo-optical material at the heating laser beam wavelength. This change is obtained by modifying the intensity of the additional, flat in profile, light of a wavelength different than that of the heating laser beam. The change in intensity of this light may follow from the effects of external factors, as, e.g., decrease in daylight intensity. Therefore, the lens may be considered as a passive auto-tuning lens, with focal length auto-adjusting to the external illumination.