ISOPLANATIC CONDITION AND THE CHARACTERISTIC OF DEFOCUS IN AN OPTICAL-SYSTEM OF LOW FRESNEL NUMBER

It is well known that in a diffraction-limited optical system of sufficiently low fresnel number N the point of maximum intensity is located closer to the exit pupil. However further consideration is required for an off-axis image point. In the present paper, using the diffraction integral, we calculate the intesity distribution of an off-axis point image. From these numerical results and theoretical consideration, isoplanatic conditions in the optical system of low Fresnel number are derived. Moreover, comparing OTFs, we discuss the best image plane numerically and theoretically.