The Strehl Definition (S. D.) is one of commonly used criterion for evaluating the quality of an optical system, especially, for the mircosoopic objectives. In this paper several problems, which play important role in S.D. calculations, are considered in detail. Namely, the defoous, the expansion formula of the wave aberrations of the deoenteral system as well as the numerical calculation method. The proposed method can also be used for estimating the quality of the decenteral optical systems.