An imaging unit is arranged in a motor vehicle. The imaging unit is designed for providing, as a function of an image acquired by it, a digital source image of a predefined image size. A first intermediate image of a predefined first intermediate image size is generated by reducing a resolution of the source image for the sake of reducing pixels. Furthermore, a second intermediate image of the predefined image size is generated such that it comprises the first intermediate image. The second intermediate image is analyzed by a predefined detector in order to examine whether an object of a predefined object category is situated in the second intermediate image, the detector being designed for analyzing a predefined image detail and for detecting an object of a predefined object category of a predefined object size range.