An improved formula for division of optical power in an achromatic hybrid refractive diffractive singlet with small field of view is presented as the diffractive optical element (DOE) is used to correct both axial chromatic and spherical aberrations. The examples show that using the revised formula one can get a better original configuration without the siganificant level of spherichromatism aberration introduced by the aspheric term of DOE for correcting the spherical aberration. The correctness and effectiveness of the formula have been proven. This formula is also useful in designing other hybrid optical systems.