Rotationally symmetric optical imaging systems have been considered which means that, the diffraction effects along with aberrations limit the performance of the optical systems. A rotationally symmetric optical system is that system, which has the same properties on the circumference of any circle and whose center lies on the symmetry axis of the system. Further it is also assumed that the optical system is isoplanatic (the imaging properties do not change over a certain distance in the image plane) where, if the object point changes in its location, the image point changes only in its location but not in functional form. The diffracted field characteristics have been studied for circular apertures in the presence of defocus under the influence of intensity variation to assess the performance of the optical imaging systems. Throughout this work rotationally symmetric optical imaging systems have been considered which means that, the diffraction effects along with aberrations limit the performance of the optical systems.