A method of imaging a sample including biological cells, the method comprising: detecting light transmitted through the sample for a first set of focal positions to collect a first stack of images; calculating values of a focus metric for the first stack of images; determining a candidate focal position based on the values; and detecting (158, 160) photoluminescence from the sample for a second set of focal positions to collect a second stack of images, including a step of detecting photoluminescence from a predetermined number of focal positions to collect the second stack of images, wherein the second set of focal positions defines a smaller range than the first set of focal positions, and wherein at least one focal position of the second set of focal positions is based on the candidate focal position, characterized by the step of detecting (158, 160) photoluminescence from the sample for a second set of focal positions further including: a step of determining whether a contrast peak is identifiable from the second stack of images; and a step of adding one or more focal positions to the predetermined number of focal positions to create an expanded second stack of images, until a contrast peak is identifiable from the expanded second stack of images, if the contrast peak is not identifiable from the predetermined number of focal positions; wherein the method further comprises: a step of determining a best theoretical focal position at an apex of the contrast peak; and a step of comparing the best theoretical focal position to a closest focal position of the second stack of images and expanded second stack of images, wherein, if the best theoretical focal position is not within a threshold proximity to the closest focal position, adjusting to the best theoretical focal position, and collecting at least one additional photoluminescence image. detecting light transmitted through the sample for a first set of focal positions to collect a first stack of images; calculating values of a focus metric for the first stack of images; determining a candidate focal position based on the values; and detecting (158, 160) photoluminescence from the sample for a second set of focal positions to collect a second stack of images, including a step of detecting photoluminescence from a predetermined number of focal positions to collect the second stack of images, wherein the second set of focal positions defines a smaller range than the first set of focal positions, and wherein at least one focal position of the second set of focal positions is based on the candidate focal position, characterized by the step of detecting (158, 160) photoluminescence from the sample for a second set of focal positions further including: a step of determining whether a contrast peak is identifiable from the second stack of images; and a step of adding one or more focal positions to the predetermined number of focal positions to create an expanded second stack of images, until a contrast peak is identifiable from the expanded second stack of images, if the contrast peak is not identifiable from the predetermined number of focal positions; a step of determining whether a contrast peak is identifiable from the second stack of images; and a step of adding one or more focal positions to the predetermined number of focal positions to create an expanded second stack of images, until a contrast peak is identifiable from the expanded second stack of images, if the contrast peak is not identifiable from the predetermined number of focal positions; wherein the method further comprises: a step of determining a best theoretical focal position at an apex of the contrast peak; and a step of comparing the best theoretical focal position to a closest focal position of the second stack of images and expanded second stack of images, wherein, if the best theoretical focal position is not within a threshold proximity to the closest focal position, adjusting to the best theoretical focal position, and collecting at least one additional photoluminescence image. a step of determining a best theoretical focal position at an apex of the contrast peak; and a step of comparing the best theoretical focal position to a closest focal position of the second stack of images and expanded second stack of images, wherein, if the best theoretical focal position is not within a threshold proximity to the closest focal position, adjusting to the best theoretical focal position, and collecting at least one additional photoluminescence image.