An auto-focus image system that includes a pixel array coupled to a focus signal generator. The pixel array captures an image that has a plurality of edges. The generator generates a focus signal that is a function of a plurality of edge-sharpness measures, each being measured from a different one of the plurality of edges. The edge-sharpness measure is a quantity that has a unit that is a power of a unit of length. It may be a distance in the edge. It may be an area. It may be a central moment. The generator may reduce a relative extent to which an edge contributes to the focus signal on basis of detecting that the edge does not have sufficient reflection symmetry in a sequence of gradients of an image signal across the edge according to a predefined criterion. The edge may be prevented from contributing altogether.