A fixed pattern noise of an image converter is automatically determined. A first optical image is converted using the image converter into digital image data, the light level of the first optical image is determined, the light level of the first optical image is compared with a threshold value, and the image converter characteristics of the image converter are reset to a dark image setting in such a way that the image data generated over the photosensitive imaging surface of the image converter at constant distribution of the determined light level include no component produced by the light level. Immediately thereafter, using the dark image setting previously set, a first dark image is recorded.