In this paper, we propose a new self-calibration technique for cameras with changing zoom observing only a planar structure. The method does not need any metric or topologic knowledge about the structure since it is based on the estimation of the collineations existing between several views of a plane (thus only image correspondences are needed). The constraints existing between all the collineations are imposed using a very simple and efficient technique which does not need the solution of a complex optimisation problem. Finally, even if the structure of the plane is unknown it must be the same for all the images and this provides some constraints which allow the recovering of the varying focal length.