PROBLEM TO BE SOLVED: To provide an image forming optical device in which a high resolution observation is possible under an optical system including a spherical aberration and a chromatic aberration by eliminating the influence of both aberrations.
SOLUTION: In an image forming optical device having the spherical aberration and a chromatic aberration and using a wave beam [electromagnetic wave of various wavelength bands including light wave, sonic wave (ultrasonic wave), electron beam, charged particle beam, or the like], an observed images are integrated while continuously varying a focus deviation amount within a prescribed value (equivalent to averaged movement of a focus and chromatic aberration). When the variable range of the focus deviation amount is sufficiently larger than a value which is determined by a spherical aberration coefficient, the integrated images have a deeper depth of focus and is not influenced by the spherical aberration (a rotationally invariant wavefront aberration) of the optical system. However, since in this case, the intermediate and high spatial frequency components are suppressed in the integrated images, the intermediate and high spatial frequency intensifying filtering is applied to recover the components to reproduce a clear image. Thus, the influence of both of the spherical and chromatic aberrations is eliminated and accordingly the observed image having a deep depth of focus is reproduced.