1. A method of determining an optimal threshold value (t O ) for an image, said method comprising the steps of: a) obtaining an image; b) selecting a test segment of said image; c) determining the mean feature size (S) of features appearing in said test segment of said image at each of a plurality of threshold values (t), so as to produce mean feature size data (S(t)); d) selecting a subset of the mean feature size data gathered in step c); and e) determining an optimal threshold value (t O ) as a function of said subset of mean feature size data.