A beam of laser light enters the camera. The light diffusely reflected by the film re-emerges, and the speckle pattern formed at the plane of the lens diaphragm is observed. A traversing lens produces slight movements of the light spot at the film plane. If the camera is out of focus these cause bodily movements of the speckle pattern. As the focal position is approached the speckle size and rate of movement increase and finally give way to a random change of structure. The focal position can be estimated to within 0.1 or less of the Rayleigh limit. Using this it is possible to measure all the primary and secondary aberrations (other than distortion) of the camera, including any errors of film position.
L. H. Tanner, "Camera Testing by Use of Speckle Patterns," Appl. Opt. 13, 2026-2034 (1974)