The point spread function of a static fiber bundle is not spatially invariant, and the system cannot therefore be strictly characterized by an optical transfer function. It is shown both theoretically and experimentally that, in the case of circular fibers of diameter d, the modulation of the image of an object varies with the position of the fiber bundle and lies between two extreme values which become closer together as the spatial frequency decreases. Thus, for sufficiently low frequencies (of the order 1/8d), an optical transfer function can be used. The experimental results are in good agreement with the theory. A randomly vibrating bundle is found to have a spatially invariant point spread function and can therefore be characterized by an optical transfer function.
RENEE DROUGARD, "Optical Transfer Properties of Fiber Bundles," J. Opt. Soc. Am. 54, 907-907 (1964)