We present a method for measuring focus aberrations on wide field telescopes based on an entropy analysis of a single image. First, we calibrate the system using the evolution of the entropy as a function of the position in the field and the focuser position. This gives us a model defining the tilt of the sensor and the field curvature. Then, using a single image at a given position of the focuser in which the mean defocus is unknown, we can compute the position where the focuser must be set in order to minimize the focus aberration over the whole field.