Out-of-focus images tend to move and split when obstructions are passed through or near the nodal planes of the image forming lens, while in-focus images do not show such effects. Geometrical optics is used to produce a qualitative explanation of these effects with the object of utilizing them for automatic focusing and rangefinding devices. Several problems in such devices are briefly discussed and a proposed automatic rangefinder is described.
W. J. Harrison, "Out-of-Focus Images; Some Properties, Uses, and a Geometrical Explanation," Appl. Opt. 6, 1559-1559 (1967)