Abstract: Apparatus and methods are described for use with a cell sample that includes a plurality of cells. A series of images associated with a series of depth levels of the cell sample are acquired, by performing a depth scan of cell sample with a microscope. One of the depth levels is identified as being an optimum focal plane for imaging one or more entities within the cell sample using the microscope, under a first illumination condition. The cell sample is imaged under a second illumination condition that is different from the first illumination condition, using the microscope, by focusing the microscope at an investigative depth level that is based on the identified depth level. Other applications are also described.