Optimized image formation is an essential step preceding automatic image analysis. We report about an optical device for automatic prescreening of cervical cytologic samples. The system should detect abnormal cells. Using band-pass filtering in the Fourier plane a detection of abnormal cells is possible. The main advantage of this method is its ability of parallel processing in real time. That means we can handle a large object field at once in a microscope. We use a very simple parameter -the size of the cell nucleus- for a discrimination between normal and abnormal cells. As a typical result the detection of abnormal cells in a Papanicolaou smear by band-pass filtering is showed.