Optical transfer theory is not applicable to objects that have an extension parallel to the optical axis. In this paper a transfer theory is developed that applies to the “longitudinal object,” defined as a radiant line source that is directed in an arbitrary field direction θ. The “longitudinal image,” defined as the irradiance along the resulting gaussian image, is Fourier analyzed, resulting in a “longitudinal transfer theory.” This theory is predicated on the existence of a line of stationarity, along which the longitudinal object must lie. Stationarity is verified in the diffraction-limited case. One result of the theory is a new method for computing the depth of focus.
B. ROY FRIEDEN, "Longitudinal Image Formation," J. Opt. Soc. Am. 56, 1495-1501 (1966)