In the optical system, the conjugate transformation of the light field between object and its image is a common transformation. Based on the paraxial approximation and the Fresnel diffraction theory, the relation of light field between the object plane and its conjugate image plane of lens is presented in the book of Introduction of Fourier Optics. It is consistent with the scale transformation of the geometric optics prediction. The sine condition is a requirement for lens which carries out a perfect image for the object with vertical axis facet in an optical system. It was demonstrated by Fermat's principle in the field of geometrical optics, but it could not be explained by the classical diffraction integral formulae in the non-paraxial field.As the law of conservation of radiant energy in the traveling wave field was underappreciated in the classical diffraction theory, the classical diffraction integral formulae have limitations. As the Huygens' principle and the law of conservation of radiation energy in traveling wave field were engaged for analyzing the diffraction process of the non-paraxial light field in the cylindrical coordinate system, it was presented that the rationality of the normalized inclination factor was expressed by the square root of the cosine of the inclination angle in the side of diffraction source for analyzing the far field characteristics what diffracted from small light fields with flat wave front, and it was presented that the rationality of the normalized inclination factor was expressed by the square root of the cosine of the inclination angle in the observation side for analyzing the characteristics of the light field in the focal plane what diffracted from convergent hemispherical wave front also.Based on the normalized inclination factor expressed by the square root of the cosine of inclination angle, the non-paraxial integral formulas for analyzing the far-field characteristics of the diffraction source with the symmetric small flat wave front and the focal plane field characteristics of the diffraction source with the symmetric convergent wave front are suggested in the rectangular coordinate system respectively. As the light field of the diffraction source is a symmetric traveling wave field, and the Parseval's theorem and the calculation formula of the total power of the light field on the reference surface are employed, the total power of the light field on the observation reference surface which was expressed by the new normalized inclination factor equal to the total power of the light field on the diffraction source reference. It satisfies the law of conservation of energy. So the new normalized inclination factor is reasonable.As the Huygens' principle is engaged for analyzing the diffraction process of the symmetric traveling wave field, both the diffraction source reference surface and the observation reference surface coincide with the light wave fronts. The diffraction source reference surface is a small flat wave front, and the observation reference surface is a hemispherical wave front for analyzing the far field characteristics what diffracted from small light fields with a flat wave front. The diffraction source reference surface is a hemispherical wave front, and the observation reference surface is a small flat wave front for analyzing the characteristics of the light field in the focal plane what diffracted from the convergent hemispherical wave front. These two diffraction processes are the process of exchanging the diffraction source reference surface and the observation reference surface. If the diffraction source reference surface and the observation reference surface were reciprocated, the expression function of the traveling wave field of symmetric diffraction source and the expression function of the traveling wave field of observation surface in two diffraction integral formulae are reciprocated, and the functions of two inclination factors in two diffraction integral formulae are reciprocated also. Then, the reciprocity between the non-paraxial diffraction integral formula used to calculate the far field distribution of light wave what diffracted from symmetric small traveling wave field with a flat wave and the non-paraxial diffraction integral formula used to calculate the light field distribution of focal plane what diffracted from symmetric traveling wave field hemispherical wave front is clarified.As the normalized inclination factor of diffraction source side expressed by the square root of the cosine of inclination angle of diffraction source side and the normalized inclination factor of observation side expressed by the square root of the cosine of inclination angle of observation side are engaged, and the apodization function of the lens on the normal of light wave front is taken into consideration. The scale transformation characteristic of traveling wave fields with flat wavefront between the object-image conjugate planes of the lens which satisfied the sine condition is verified, and the applicable conditions of the relationship of scale transformation are expounded.