In general, the work of optical design is to minimize Seidel aberrations. If the lens designer lacks the fundamental theory of aberrations, he may face tedious aberrations correction processes. However, nowadays powerful lens design software can give a quick answer to this specific purpose. The purpose of this paper is to present the analytical calculations of the first primary aberration, i.e., spherical aberration, by paraxial and meridional raytracing. By analyzing the spherical aberration of the singlet, a rigid concept of how the aberration is calculated by computer from the basic ray tracing technique is given. Thus helping to obtain a better structure of first order lens design.