Auto-focus image system

A method for generating a focus signal from a plurality of edges of an image of a scene to indicate a degree of image sharpness, comprising:

• evaluating in a computing device a first measure and a second measure on an edge detected from the image to find a first value and a second value, respectively;
• and,
• determining by use of at least the first and second values a relative extent to which the edge weighs in contributing to the focus signal as compared with other edges that contribute to the focus signal,
• wherein the first and second measures of any edge are each a quantity that depends on at least two image-sample differences, each image-sample difference being a difference between a pair of samples of image data, the samples being from a sequence of image data samples across said any edge,
• wherein the determining is not based on measuring an extent to which a sequence of gradients across the edge lacks reflection symmetry.