Light field cameras can record the spatial and angular information of light rays at the same time, which facilitate many vision tasks such as 3D reconstruction and digital refocusing. In this paper, we deduce a projection model from a 3D space point to each sub-aperture image according to the transmission of light rays in the light field camera, and the projection model of traditional cameras can be considered as a special case of it. We propose a camera calibration method using this model. Firstly, the homography from a space plane to each sub-aperture image is estimated, which implies the imaging parameters of the standard light field camera, and then the principal line on each sub-aperture image is derived from the constraint of the fixed point of the plane homography. The principal point of each sub-aperture image is robustly determined from the principal lines by using RANSAC. The other parameters can be determined from the homography constrains on them. Compared with the state of art methods, the proposed method can calibrate the light field images with a varying focal length and has higher accuracy. Moreover, it provides a unified framework for the calibration of standard light field cameras and traditional cameras. Experimental results validate the proposed method.