The light field imaging with microlens array is modeled through analyzing three fundamental factors, such as space, lens, and sensor. The spatial multiplexing is explained and the relationship between the light field and the pixel in raw image is described. Based on the phase shifting, a calibration method is proposed to determine the centers of microlens, the corresponding relationship between the pixel and the microlens, and the character of the microlens vignetting. The experiment is conducted on a commercial light field camera, Lytro. The light field is decoded from the raw image and the refocus image and all-focus image is created from the obtained light field. The experiment results confirm the proposed model and calibration method is valid.