US Patent 7280149 Method and apparatus for detecting optimum lens focus position

The present invention is a technique for measuring optimum focus position of a lens to include the steps of capturing a digital image in the field of view, digitally compressing the image to provide a compressed file having a file size and providing a focus accuracy parameter derived from the file size of the compressed file. The key point of this invention is the recognition that the compressed file associated with a given image increases monotonically in size as the image is brought into focus. The latter occurs because a focused image has greater information content than an unfocused image. With such a technique, an optimal focus position of a lens can be determined by maximizing a single parameter, the compressed output file size.