1. A method for determining the quality of focus of a digital image of a biological specimen comprising: (a) obtaining a digital image of the specimen using the specimen imaging apparatus, the digital image comprising a plurality of pixels; (b) calculating a measure of small-scale texture in the image, wherein the small-scale texture is measured by squaring the differences of the means of adjacent pairs of pixels across the entire digital image; (c) calculating a measure of larger-scale texture in the image, wherein the larger-scale texture is measured by squaring the differences of the means of adjacent triples of pixels across the entire digital image; and (d) establishing an estimate of the image focus quality based on a comparison of the small-scale and larger-scale texture measurements.