about 1/2way down this page you will see a zone plate.
it looks like a "bullseye" , and can give you a beautiful glowing image.
if you have a lubitel 166 ... ( the tlr lomo )
the zone plate will fit perfectly in front of the shutter.
pinholeresource sells zone plates and laser - drilled pinholes
sorry i didn't really answer your question ..
i was going for the picture is worth 1000 words