Originally posted by IgorZ Thanks for the suggestion, I'll check out Cokin. From what I understand, the disks used with mamiya lenses are just pieces of metal with one big hole in the center and a bunch of small holes around it.
Were these disks mounted where lens filters are usually mounted, in front of the objective lens? What a hole pattern would be expected to generate (located at the effective pupil) is a complex diffraction pattern different from the diffraction limited Airy disk pattern that usually results from setting the f/no to a high value. Solving a nightmare of equations would yield the resulting diffraction pattern from an arbitrary hole pattern. I have never seen a paper that described hole patterns that would be good for slightly diffuse portraiture. I also cannot imagine what modulation transfer function would be superior for this task relative to just a low-pass filter. But what I imagine results is the potential for narrow depth of field (from low f/no) plus blur within the DOF that looks different than spherical aberration or whatever. Velly intellesting approach.
I wonder if this is treated to some extent in
Introduction to Statistical Optics by Edward L. O'Neill. I see this has become a Dover book, so I'll have to get a copy next time I order some books.
---------- Post added 23rd Nov 2018 at 17:48 ----------
I dug out the Cokin filter. To the casual viewer it would look clear and transparent. Close inspection reveals a random pattern of "microscopic worm tracks" on one side of the plastic. I believe these changes in thickness cause different rays from the same [far field] point to have different random phases at the focal plane that will subtly degrade the image resolution. This might be imagined as tiny amounts of weak narrow-angle forward scattering applied to the image. A multiple hole aperture would also create both image diffraction but also phase effects from the offset holes. I suspect the results are different in how each portrait effect looks macroscopically.
The Cokin filter designation is Diffuser 1 P. 083
Last edited by kaseki; 11-24-2018 at 10:21 PM.