William -- NEX-5 & Paillard-Bolex 50mm f1.3 Projectioin Lens
So I tried taping an aperture disc made from thin black plastic and placed it at the end of the lens. Didn't seem to affect the depth of field much or affect the image quality much either. The position of the aperture disc is probably not the right place for aperture control, but unfortunately, it's either in front or at the back of the lens. Perhaps I shall try it in the front instead, like the Sima 100mm f2 soft focus lens.