1. A measurement apparatus for enumeration of white blood cells in a sample, the apparatus comprising: a holder, which is arranged to receive a sample acquiring device comprising a measurement cavity that holds a sample, wherein the measurement cavity is adapted to hold a stained and hemolysed sample, and wherein the measurement cavity has a uniform depth of 50-200 micrometers, an imaging system adapted to acquire a plurality of digital images of the sample at different levels in a direction of depth of field in the sample using different optical settings, wherein the imaging system is arranged to obtain said digital images with a depth of field in the range of 2-30 micrometers, and an image analyzer, which is arranged to analyze each of the acquired plurality of digital images for identifying stained white blood cells and determining the number of white blood cells in the sample, wherein the image analyzer is arranged to analyze each of the acquired plurality of digital images for identifying white blood cells that are imaged in focus and determining types and number of the white blood cells that are imaged in focus, the types being distinguished by physical features of the stained white blood cells, whereby the ratio of different types of white blood cells in the sample is determined; wherein a white blood cell in an image is determined to be in focus when a value of a property of the image white blood cell falls within a predetermined range; wherein the image analyzer is arranged to, for a specific white blood cell, determine the number of said images in which said white blood cell is imaged counting from an image in which the white blood cell is determined to be out of focus in a first direction, to an image in which the white blood cell is determined to be out of focus in a second direction; wherein determining the types and number of the white blood cells that are imaged in focus is based at least in part on the number of images.