This paper studies the multi-image calibration of a camera and how to carry it out in an automatic way making use of a projector. To prepare for multi-image calibration, it is necessary to calibrate the camera. Next, calibrate the projector using that camera. Then calibrate any camera with similar focal length to the previous one. The system doesn't need to incorporate any position platform and is completely automatic once it is disposed. It is based on the generation of images of a virtual pattern. This virtually generated pattern is projected on a fixed blackboard or a wall. Different on-line generated virtual patterns are visualized to carry out the auto-calibration. It is simple and can be repeated for a number of cameras that have an appropriate focal length. Also an improved method for focus of expansion using the same ideas is presented to obtain the principal point for a zoom camera.