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; (c) calculating a measure of larger-scale texture in the image, and (d) establishing an estimate of the image focus quality based on a comparison of the small-scale and larger-scale texture measurements.