Disclosed is an optical system that has superior optical characteristic and wear resistance and can be formed at low temperatures. An optical system 101 for adjusting visible light transmittance to have a desired value comprises fluoride 103, and at least part of the optical system has a crystal grain diameter of 3 nm to 10 nm. The fluoride 103 has a specific surface area of 1 m.sup.2/g to 5 m.sup.2/g.