1. A method for determining the ventilation of a lung of an object under examination by means of magnetic resonance imaging, comprising the steps of operating a magnetic resonance data acquisition unit to acquire at least two first lung-representing image data sets at different intervals of the breathing phase; in a computerized processor, automatically determining a density change of the lung tissue by determining a signal difference from the image signals of the first image data sets in at least one corresponding region of the first lung-representing image data set; in said processor, automatically determining a lung volume or a thorax volume using at least two of the first image data sets, or using at least an additional second lung-representing image data set in a breathing phase which corresponds with the breathing phase of a first image data set; and in said processor calculating the localized, quantitative ventilation of the lung, depending on the density change of the lung tissue and the change of the lung or thorax volume.