Based on the Huygens-Fresnel principle, spatial frequency spectrum theory, and the law of energy conservation, we derive an inclination factor of the integral formula for far-field diffraction from a small diffractive source with a planar wavefront and provide an expression to calculate the light field distribution on the far-field hemispherical surface. Then, according to the characteristics of the wavefront transformation of a collimating objective satisfying the sine condition, a calculation formula is recommended for a collimating light field from a small diffractive source with a planar wavefront. Furthermore, applications of this formula are shown through the transformation characteristics of Gaussian beams of simple rectangular coordinate polarization and simple cylindrical coordinate polarization.