An apparatus configured to expose a substrate to light includes an illumination optical system configured to illuminate a mask, a projection optical system configured to project a pattern of the mask onto the substrate, and a decentering mechanism configured to decenter at least one optical element of the illumination optical system with respect to an optical axis of the projection optical system, or to decenter at least one optical element of the projection optical system with respect to an optical axis of the illumination optical system, and rotational asymmetry distortion that occurs at a position defocused from a focus position of the projection optical system is changed by decentering the optical element by the decentering mechanism.