A projection optical apparatus for projection an image of an object onto a workpiece, which is suitable for use, for example, as an exposure apparatus used in the manufacture of integrated circuits. The projection optical apparatus includes an image forming optical system whose optical characteristic is changed by the light energy supplied from the illuminated object. A change of the optical characteristic of the optical system is determined by the use of a predetermined parameter whereby when the distribution of the light energy on the pupil of the optical system is changed, the parameter is correct in such a manner that the change of the optical characteristic determined by the parameter is changed in correspondence to the change in the distribution of the light energy.