I think the idea with an Imagon is always to use the aperture wide open and use the disks to control exposure.

The principle of the "sink strainer" aperture is that the softness is created by spherical aberration, or the difference in focus between rays that enter at the edge of the lens as opposed to rays than enter near the lens axis. Normally this would be corrected as you stop down, because you are reducing the contribution of light from the edge of the lens, but the disk allows you to reduce the amount of light entering the camera while still using the rays from the edge. Some soft focus lenses achieve this effect by using a star-shaped diaphragm, rather than a round diaphragm.