This work presents a discussion about quantification techniques of the changes in individual and total monochromatic / chromatic wavefront aberrations when changes in constructive parameters are done as an intermediate step in tolerancing. We propose a technique based in the wavefront aberrations polynomial and in the evaluation of their coefficients in contrast to one that is fully bases in rss technique for adding wavefront aberrations. We apply these two techniques for analyzing the effects in the performance of optical systems due to changes in manufacturing tolerances. We apply these results to find axial critical constructive parameters in the case of a low vision telescope. Also, it is shown how to decrease the sensibility of critical elements.