Rodenstock stated in one of the Imagon brochures that combining the area of all of the smaller holes (wide open), plus the central opening, does not yield the same opening as the computed f stop for the focal length... thus the term "H" (Helligket or lightness) instead of "f". The difference according to Rodenstock is slight (not quantified) but a difference nonetheless. I hope I am not confusing folks needlessly and I can't add more because that is all the information I was able to obtain from a lot of really old German technical notes on the Imagon. Someone with better math skills and understanding of diffraction/refraction can probably calculate the difference between "H" and "f" values.
Joel