Nice setup ! So you have a separate helicoid with an aperture-ring, to stop down the lens?
Such lenses and also projection lenses are usually used open, as they have no aperture.
Some people either perform some (delicate) surgery and add one (I have a few done by a very skilled mechanic),
or use an added aperture after the lens (enough back focal permitting), but that often leads to vignetting). A front
aperture is possible too, but for large lenses bit difficult to make and use.... But why bother, the idea is to USE
such a lens for its BG separation, requiring the full opening to achieve that.
