A method of producing a multiple focus stack image of a target, the stack image comprising a plurality of images of the target, each image having a corresponding focal range or position, wherein the method comprises causing relative scanning movement between the target and an array of light detectors, the array being used to repeatedly receive image information from the target during the scan in the form of scan lines; and causing the relative focus between the target and the array to be modified between the said focal ranges or positions of the respective images during the scan so as to obtain scan lines corresponding to each focal range or position during the scan, thereby obtaining the images in the stack, wherein each image is formed from the image information obtained as scan lines at the respective focal range or position during the scan. The method is then further characterised in that the relative focus is cycled between two extreme focal ranges or positions during the relative scanning movement and at least one scan line is obtained at a focal range or position located between subsequent extreme focal ranges or positions during the scan, each scan line being used to form one of the images of the target