There is good reason to suppose that the properties of the three element lens are universal in the weak lens limit. In this paper we first derive the lens properties in that limit, particularly the focal length and the two principal abberations, chromatic and spherical. Next we examine these same properties which are computed for some real lenses in practical conditions and compare them with the weak lens predictions. The agreement is found to be remarkably good, indicating that simple calculations can be used to estimate lens performance.