The shape occurs with a wide open lens. I have an N8008S and that's the one it happens with. With the F3 I just get the standard realistic blur. Looking at a specular highlight, or for example a small LED light on a printer, and defocusing, what I get is a pentagon with a darker center. The center has a lighter spot inside it. Light and dark lines run through the pentagon extending beyond it, making its edges soft. When focused closer than the object the pentagon is flat on the bottom, when focused farther it inverts. An out of focus straight line, like a tree trunk or roofline of a house appears as a double line, though the lens does not render it that way.
All very "prismatic".
Stopped down the shape of the diaphragm "trims the edges" of the pentagon and the diaphragm shape prevails. The other characteristics remain, diminishing as depth of field brings the image closer to focus.