In order to understand the image formation inside plenoptic systems, a wave-optic-based model is proposed in this paper that uses the Fresnel diffraction equation to propagate the whole object field into the plenoptic systems. The proposed model is much flexible at sampling on propagation planes by utilizing the method of multiple partial propagations. In order to verify the effectiveness of the proposed model, numerical simulations are conducted by comparing with existing wave optic model under different optical configurations of plenoptic cameras. Results demonstrate that the proposed model can describe the light field image formation properly. In addition, the time for image formation has been reduced by a factor of 19.22 using the proposed model.