Relationship between frame entropy as the focus level changes in the z-stack is shown in (a) and (b). In (a) there is a monotonic increasing and then decreasing relationship between focus and entropy, with the in-focus frame containing minimum entropy. In (b) a nosier data set is employed and the relationship between focus and entropy is irregular. As can be seen in frame 6, banding and stripe noise introduced by the microscope unexpectedly increases entropy. (c) Illustrates four corresponding frames for data set analyzed in graph (b).