Wow! Three pinholes would certain increase the amount of fooling around. I guess if you are into research primarily, it would be worth it, but if you are planning to make images, you'll make a lot less of 'm. In my experience, for practical purposes, I really doubt that even the most discerning viewer - i.e. anyone who isn't looking at your work through an electron microscope - would ever notice the difference between three color images made with a pinhole calibrated to a middle value. That's usually in the green part of the spectrum... I'm sure I could dredge up a wavelength if someone really needs it, in my collection of Pinhole Journal.

Usually, pinholes aren't made to sufficient quality standards where wavelength variation would be the main source of degradation. Most people seem to be using (if I'm not wrong) laser drilled pinholes. In these, only the center has the nominal diameter. As you move away from the center of the image, not only the size of the opening changes, but its aspect as well, and additional diffraction elements would enter into the picture.

Using a constant d and varying the "focal length" (misnomer, but you get what I mean) would probably not be what Bill has in mind, because each color image would have a slightly different angle of view and objects rendered in different scales. Changing out the pinholes would produce consistency at the cost of adding a great deal of trouble (IMHO). Bill asked about "maximum" ph size. I have used diameters ranging between about 5/1000's of an inch to about 1 inch, depending upon how big the camera is. The 1" hole was used in a room set up as a camera. In one of his books, Ansel Adams suggested making a viewing pinhole of about 1/8" which would be replaced with a working pinhole of much smaller size. Of course, the 1/8" hole would be big blurry, but would allow recognition of general shapes. What this might mean for you, Bill, is that you could use any size hole you want, depending upon your quality requirements. You could use a hole so big that you'd have a great shutter speed, but might sacrifice a recognizable image in achieving that.