Extending the depth of field through unbalanced optical path difference

We describe a simple method to extend the depth of field of a conventional camera by inserting a transparent annular ring in front of the pupil of the lens. The insertion of the ring creates an unbalanced optical path difference across the lens aperture, which partitions the pupil and leads to an extended depth of field. This system is analyzed by diffraction and random process theory. Experiments are reported that are in good agreement with the theory.