The aperture size of microlens is a key parameter of a plenoptic camera, which strongly influence the performance of the camera. Generally, the size of microlens aperture is submillimeter, which will introduce obvious diffraction effect with large f-number. Besides, optical distortions of plenoptic camera also bring degradation effect in raw plenoptic image. In this letter, based on wave optics, a more accurate imaging model with diffraction effect and optical distortion for focused plenoptic camera is proposed. The modulation transfer function (MTF) evaluation suggests that the proposed model is more close to the real one when compared to the previous models. Additionally, the raw images under different microlens aperture sizes are synthesized. Finally, a relatively optimal size of microlens aperture is obtained according the results of depth estimation.