What does mounting a pinhole or zone-plate over a lens, in theory, do?

This might be a dumb question. I shoot quite a bit of pinhole and zone plate imagery directly on to film. One of my film pinhole cameras takes a variety of pinholes and zoneplates which are mounted on 52mm filter rings.

52mm filter rings happen to be the standard size for most of the ai Nikon manual focus lenses. What effect, in theory, does putting the pinhole or zoneplate over the lens have?

I tried a 33 zone plate mounted onto a Nikon 105mm lens. The results are more soft-focus than zone plate. Much different, as I would expect, from a zone plate image only.

Too much vignetting on a 50mm lens.

Placing a zone plate over the Nikon lens results in lack of sharpness to both diffraction and to the zone plate acting like a diaphragm in the wrong position along the optical axis. Theory can be complex in this situation. Rather than think about theory, experiment.