An auto-focus image system includes a focus signal generator and a pixel array coupled thereto that captures an image that includes a plurality of edges. The generator computes a focus signal from a plurality of edge-sharpness measures, each measured from and contributed by a different edge as a quantity with a unit that is a power of a unit of length, such as a distance in the edge, an area, or an even-order central moment. A relative weight of the contribution by an edge is reduced depending on at least a pair of shape measures, each being computed from a plurality of sample-pair differences of the edge. One may be the edge-sharpness measure. The weight may be zero if the pair of shape measures falls outside a predetermined region. At least one symmetrical sequence of gradients exists such that an edge with it has reduced relative weight.