In multiview (MV) or super multiview (SMV) glasses-free 3D displays, depth of field (DOF) is one of the most important parameters. The greater the distance of the displayed object outside or inside the screen, the more blurred it will be; therefore, a larger value of DOF can give more depth information and more pleasing 3D images. In this paper, we will first analyze how the voxels, which are the 3D equivalent of pixels in a conventional display, are formed in space and how it affects DOF; we will then show how to increase DOF by increasing the focal length of the lenticular lens. In our experiments, we made two 3D displays using two lenticular lenses with the same parameters, except for the focal length, and showed that the larger the focal length of lenticular lens, the larger the DOF of the 3D displays. However, for the same 2D screen, the larger DOF results in a smaller viewing angle. We then applied eye-tracking to increase the viewing angle by a factor of about three times in order to compensate for the reduced viewing angle.