1. A method for optical self-diagnosis of a camera system adapted to record a stereo image obtained from at least two partial images, the method comprising: a) recording a first partial image and a second partial image of an object space with the camera system from different perspectives; b) producing a depth image from said first partial image and said second partial image on the basis of parallax between mutually corresponding pixels recorded in said first and second partial images, and determining disparity values representing said parallax; c) determining a number of valid disparity values of the depth image, wherein a valid disparity value is defined as a disparity value which is determined for two pixels, recorded in the first and second partial images, which are identified as being mutually corresponding either unequivocally or at least with a defined probability; d) producing a warning signal depending on the number of said valid disparity values, wherein a comparison of the number of valid disparity values is performed with a predetermined threshold value, and the warning signal is produced when said number of disparity values falls below the predetermined threshold value, wherein several disparity maps are calculated and compared with each other over a period of time, with the warning signal being produced only when a number of valid disparity values in the calculated disparity maps falls below the predetermined threshold value of valid disparity values within said period of time.