DETERMINING DEFOCUS IN IMAGE DATA RELATED TO A PREPARED BLOOD SAMPLE

A method for determining defocus in image data related to a prepared blood sample, by means of a blood analyser, is disclosed. Image data related to a prepared blood sample arranged in a probing volume is obtained by means of an imaging system, the image data comprising data related to at least one imaging plane corresponding to a depth of the prepared blood sample. The image data is analysed by identifying an object region in the obtained image data, the object region comprising a group of pixels in the image data which corresponds to an object being physically present in the prepared blood sample, and identifying an optical feature of the identified object region, the optical feature originating from a difference in refractive index between the object and a medium of the prepared blood sample, and the optical feature acting as an additional artefact in the image data that does not represent an object being physically present in the prepared blood sample. At least a direction of defocus in the image data is determined, based on an appearance of the identified optical feature.