Interesting. Past f:8 or so, most soft lenses lose the spherical aberration that is largely responsible for the soft effect, and by f:22 or so diffraction becomes enough of a limitation that there isn't too much difference between lenses in general, though there are some things that can't be improved by stopping down. The disks make it possible to reduce the amount of light entering the lens without losing the spherical aberration that causes the soft effect.