Few models, based on the diffraction theory, are proposed in order to evaluate the point spread function of different microscope objectives used in a digital holographic microscope. Because in holography the phase information is essential, a 3D amplitude point spread function (APSF), modulus and phase, is necessary, in order to properly deconvolute the 3D images obtained. Scalar Debye theory, paraxial approximation and vectorial Debye theory are used to solve the diffraction problem and the theoretical predicted 3D APSFs obtained. with these models are compared.