An optical filter includes a substrate and a filter film attached on the substrate. The filter film has alternate layers of high reflective layers and low reflective layers to form a plurality of Fabry-Perot resonators. A basic optical thickness of the high reflective layers and the low reflective layers is one fourth a central wavelength (λ0/4) of incident ray, and each of the Fabry-Perot resonators has two high reflector films and a spacer between the high reflector films. The spacers in the Fabry-Perot resonator, which is most proximal to the substrate, and in the Fabry-Perot resonator, which is most distal to the substrate, have an optical thickness respectively in a range between four times the basic optical thickness and fourteen times the basic optical thickness.