1. An apparatus for localizing a valid area of a blood smear, comprising a provider for providing at least one picture of the blood smear; a classifier for classifying pixels of the at least one picture at least into first pixels, which represent blood cells, and second pixels, which do not represent the blood cells; a selector for selecting an area of the blood smear as the valid area on the basis of a local frequency of contiguous pixel clusters of at least A min first pixels, A min being a minimum threshold value of a number of first pixels of a pixel cluster, and of a local average size of the pixel clusters for laterally distributed areas of the blood smear.