An auto focus image system that includes a pixel array coupled to a focus signal generator. The pixel array captures an image that has at least one edge. The focus signal generator may generate a focus signal that is a function of a width of the edge and/or statistics of widths of edges. A processor receives the focus signal and/or the statistics of the widths and adjusts a focus position of a focus lens. The width can be determined by various techniques including the use of gradients. An width determined along a prescribed direction is corrected for a slant of a boundary to which the edge belongs relative to the prescribed direction. The correction may use a peak gradient in the prescribed direction along the edge in conjunction with a largest gradient of a perpendicular direction within a predefined neighborhood of the peak gradient.