The analysis of point spread functions is important for the quality evaluation of imaging systems. Point spread functions are evaluated here through their moments. But two problems impede the use of this method successfully: the definition of the integration area and the assignment of adequate values to the limits of the area. The current presentation describes a method to overcome these problems: through clipping the background noise is suppressed, and the use of reduced coordinates introduces adequate scaling and normalization for the calculation of the moments. Associated density functions are defined to perform the symmetry analysis of the central disc and the ring structure of the point spread function separately. Their geometric parameters are determined from the moments and are used as symmetry descriptors. Experimental results of the evaluation of microscope objectives are presented.