1. A digital camera incorporating a sharpness predictor, comprising: an image sensor; a user interface for displaying messages to a user; and a data processing system; and a memory system communicatively connected to the data processing system and storing instructions configured to cause the data processing system to implement a method for determining-sharpness predictors for input digital images that are not test pattern images, wherein the instructions comprise: capturing an input digital image by photographing a natural scene using the image sensor; dividing the input digital image into image blocks; using a processor to compute blocks of discrete cosine transform coefficients by applying discrete cosine transforms to the image blocks; using a processor to compute one or more image metrics by analyzing only the single input digital image captured by the digital camera, wherein at least one of the image metrics is an image sharpness metric related to the spatial frequency content of the single input digital image which is determined by combining the discrete cosine transform coefficients computed for a plurality of image blocks; using a processor to compute a single sharpness predictor for the single input digital image from the one or more image metrics, wherein the sharpness predictor represents an indication of the level of sharpness degradations introduced by the digital camera; and displaying a message on the user interface when the computed sharpness predictor indicate that the input digital image lacks sufficient sharpness.